Semiconductor device and method for manufacturing the same

ABSTRACT

An object is to provide a structure of a transistor which has a channel formation region formed using an oxide semiconductor and a positive threshold voltage value, which enables a so-called normally-on switching element. The transistor includes an oxide semiconductor stack in which at least a first oxide semiconductor layer and a second oxide semiconductor layer with different energy gaps are stacked and a region containing oxygen in excess of its stoichiometric composition ratio is provided.

TECHNICAL FIELD

One embodiment of the invention to be disclosed relates to asemiconductor device and a method for manufacturing the semiconductordevice.

In this specification, a semiconductor device generally means a devicewhich can function by utilizing semiconductor characteristics, and anelectrooptic device, a semiconductor circuit, and an electronic deviceare all semiconductor devices.

BACKGROUND ART

In recent years, semiconductor devices have been developed to be used asan LSI, a CPU, or a memory. A CPU is an aggregation of semiconductorelements each provided with an electrode which is a connection terminal,which includes a semiconductor integrated circuit (including at least atransistor and a memory) separated from a semiconductor wafer.

A semiconductor circuit (IC chip) of an LSI, a CPU, or a memory ismounted on a circuit board, for example, a printed wiring board, to beused as one of components of a variety of electronic devices.

A technique for manufacturing a transistor by using an oxidesemiconductor film for a channel formation region, or the like has beenattracting attention. Examples of such a transistor include a transistorin which zinc oxide (ZnO) is used as an oxide semiconductor film and atransistor in which InGaO₃(ZnO)_(m) is used as an oxide semiconductorfilm. A technique for manufacturing such a transistor including an oxidesemiconductor film over a light-transmitting substrate and applying itto a switching element or the like of an image display device isdisclosed in Patent Documents 1 and 2.

REFERENCE

[Patent Document 1] Japanese Published Patent Application No.2007-123861

[Patent Document 2] Japanese Published Patent Application No.2007-096055

DISCLOSURE OF INVENTION

Further, it is preferable that a channel be formed at a positivethreshold voltage (V_(th)) which is as close to 0 V as possible in atransistor used in a semiconductor device. If the threshold voltage ofthe transistor is negative, the transistor tends to be in a so-callednormally-on state, in which current flows between the source electrodeand the drain electrode even when the gate voltage is 0 V.

An object of one embodiment of the present invention is to provide astructure of a transistor with a positive threshold voltage value, whichenables a so-called normally-on switching element, and to provide amanufacturing method thereof. Such a transistor is an n-channeltransistor in which an oxide semiconductor is used for a channelformation region.

Further, it is important to achieve the characteristics of a transistorclose to the normally-off characteristics even when the transistor isnot a normally-off transistor depending on the material or themanufacturing condition. Another object of one embodiment of the presentinvention is to provide a structure for achieving a threshold voltage ofa transistor close to zero even when the threshold voltage is negative,that is, even when the transistor is a so-called normally-on transistor,and to provide a manufacturing method thereof.

Further, another object of one embodiment of the present invention is toprovide a structure of a semiconductor device which achieves high-speedresponse and high-speed operation by improving on characteristics of atransistor (e.g., on-state current or field-effect mobility), and toprovide a manufacturing method thereof, in order to achieve ahigh-performance semiconductor device.

Further, it is important to reduce leakage current for achieving asemiconductor device with low power consumption because the amount ofpower consumed by the semiconductor device depends on the leakagecurrent of a transistor. Thus, another object of one embodiment of thepresent invention is to provide a structure for reducing generation ofleakage current between a source electrode layer and a drain electrodelayer (parasitic channel) of a transistor.

One embodiment of the present invention achieves at least one of theabove objects.

A transistor of one embodiment of the present invention includes anoxide semiconductor layer in which oxide semiconductors with differentenergy gaps and/or different electron affinities from each other arestacked (hereinafter, also referred to as oxide semiconductor stack).Further, the oxide semiconductor stack has a region containing a largeramount of oxygen than that of the stoichiometric composition(hereinafter, also referred to as oxygen-excess region).

For example, a transistor is formed using an oxide semiconductor stackin which a first oxide semiconductor layer and a second oxidesemiconductor layer with an energy gap different from an energy gap ofthe first oxide semiconductor layer are stacked and which has anoxygen-excess region. Here, the first oxide semiconductor layer and thesecond oxide semiconductor layer preferably have different energy gapsfrom each other, and the stacked order can be interchanged.Specifically, the energy gap of one of the oxide semiconductor layersmay be 3 eV or larger, and an energy gap of the other oxidesemiconductor layer may be smaller than 3 eV.

Note that in this specification and the like, the term “energy gap” isused to mean the terms “band gap” and “forbidden band gap”.

Further, the oxide semiconductor stack may include three or more oxidesemiconductor layers. In the case where the oxide semiconductor stackincludes three or more oxide semiconductor layers, all oxidesemiconductor layers may have different energy gaps from each other, orsome of oxide semiconductor layers in the oxide semiconductor stack mayhave energy gaps equivalent to each other.

For example, an oxide semiconductor stack can include: a first oxidesemiconductor layer; a second oxide semiconductor layer which isprovided over the first oxide semiconductor layer and has a higherelectron affinity than the first oxide semiconductor layer or a smallerenergy gap than the first oxide semiconductor layer; and a third oxidesemiconductor layer which is provided over the second oxidesemiconductor layer to cover side surfaces of the second oxidesemiconductor layer. Note that the electron affinity and the energy gapof the third oxide semiconductor layer are preferably equivalent tothose of the first oxide semiconductor layer. The electron affinitymeans an energy gap between a vacuum level and a conduction band of anoxide semiconductor. The second oxide semiconductor layer with a smallenergy gap is sandwiched between the first oxide semiconductor layerwith a large energy gap and the third oxide semiconductor layer with alarge energy gap. With such a structure of an oxide semiconductor stack,an effect of reducing the off-state current (leakage current) of atransistor can be obtained.

Specifically, the energy gaps of the first oxide semiconductor layer andthe third oxide semiconductor layer are each 3 eV or larger, and theenergy gap of the second oxide semiconductor layer is smaller than 3 eV.In a transistor including an oxide semiconductor layer, the energy gapof the oxide semiconductor layer affects electric characteristics of thetransistor. For example, in a transistor including an oxidesemiconductor layer which has a small energy gap, on characteristics(e.g., on-state current or field-effect mobility) are improved. On theother hand, in a transistor including an oxide semiconductor layer whichhas a large energy gap, the off-state current can be decreased.

With a single oxide semiconductor layer, electric characteristics of atransistor are almost determined by the energy gap of the oxidesemiconductor layer; thus, it is difficult that the transistor hasdesired electric characteristics. However, in a transistor according toone embodiment of the present invention, an oxide semiconductor stackincludes a plurality of oxide semiconductor layers with different energygaps, whereby electric characteristics of the transistor can beaccurately controlled, and accordingly desired electric characteristicscan be given to the transistor.

Therefore, a semiconductor device which meets various purposes such ashigh performance, high reliability, and low power consumption can beprovided.

One embodiment of the invention disclosed in this specification is amethod for manufacturing a semiconductor device including the steps of:forming an oxide semiconductor stack which includes a first oxidesemiconductor layer and a second oxide semiconductor layer with anenergy gap different from an energy gap of the first oxide semiconductorlayer; forming a source electrode layer or a drain electrode layer overthe oxide semiconductor stack; forming a gate insulating film over thesource electrode layer or the drain electrode layer; introducing oxygeninto the oxide semiconductor stack from a position above the gateinsulating film in a self-aligned manner with use of the sourceelectrode layer or the drain electrode layer as a mask; and forming agate electrode layer overlapping with the oxide semiconductor stack withthe gate insulating film positioned therebetween.

Another embodiment of the invention disclosed in this specification is amethod for manufacturing a semiconductor device including the steps of:forming an oxide semiconductor stack in which a first oxidesemiconductor layer, a second oxide semiconductor layer with a smallerenergy gap than the first oxide semiconductor layer, and a third oxidesemiconductor layer with a larger energy gap than the second oxidesemiconductor layer are stacked in this order; forming a sourceelectrode layer or a drain electrode layer over the oxide semiconductorstack; forming a gate insulating film over the source electrode layer orthe drain electrode layer; introducing oxygen into the oxidesemiconductor stack from a position above the gate insulating film in aself-aligned manner with use of the source electrode layer or the drainelectrode layer as a mask; and forming a gate electrode layeroverlapping with the oxide semiconductor stack with the gate insulatingfilm positioned therebetween.

In the above method for manufacturing a semiconductor device, it ispreferable that the third oxide semiconductor layer be stacked to coverside surfaces of the first oxide semiconductor layer and side surfacesof the second oxide semiconductor layer.

The third oxide semiconductor layer is formed to cover side surfaces ofthe first oxide semiconductor layer and side surfaces of the secondoxide semiconductor layer, whereby an increase of oxygen vacancies inthe second oxide semiconductor layer is suppressed, and the thresholdvoltage of the transistor can be close to zero. Further, the secondoxide semiconductor layer becomes a buried channel, whereby a channelformation region can be distanced from an interface with an insulatingfilm; accordingly, interface scattering of carriers is reduced, and highfield-effect mobility can be achieved.

In any of the above methods for manufacturing a semiconductor device, itis preferable that after formation of the gate electrode layer, dopantbe introduced into the oxide semiconductor stack in a self-alignedmanner with use of the gate electrode layer as a mask.

Further, in any of the above methods for manufacturing a semiconductordevice, an interlayer insulating film may be formed over the gateelectrode, a contact hole reaching the source electrode layer or thedrain electrode layer may be formed in the interlayer insulating film,and a wiring layer connected to the source electrode layer or the drainelectrode layer through the contact hole may be formed over theinterlayer insulating film.

Another embodiment of the present invention is a semiconductor deviceincluding: an oxide semiconductor stack which includes a first oxidesemiconductor layer and a second oxide semiconductor layer with anenergy gap different from an energy gap of the first oxide semiconductorlayer; a source electrode layer or a drain electrode layer provided overthe oxide semiconductor stack; a gate insulating film provided over thesource electrode layer or the drain electrode layer; and a gateelectrode layer overlapping with the oxide semiconductor stack with thegate insulating film positioned therebetween. In the oxide semiconductorstack, a region which overlaps with neither the source electrode layernor the drain electrode layer has a higher oxygen concentration than aregion which overlaps with the source electrode layer or the drainelectrode layer.

Another embodiment of the present invention is a semiconductor deviceincluding: an oxide semiconductor stack which includes a first oxidesemiconductor layer, a second oxide semiconductor layer which is incontact with the first oxide semiconductor layer and has a smallerenergy gap than the first oxide semiconductor layer, and a third oxidesemiconductor layer which is in contact with the second oxidesemiconductor layer and has a larger energy gap than the second oxidesemiconductor layer; a source electrode layer or a drain electrode layerprovided over the oxide semiconductor stack; a gate insulating filmprovided over the source electrode layer or the drain electrode layer;and a gate electrode layer overlapping with the oxide semiconductorstack with the gate insulating film positioned therebetween. In theoxide semiconductor stack, a region which overlaps with neither thesource electrode layer nor the drain electrode layer has a higher oxygenconcentration than a region which overlaps with the source electrodelayer or the drain electrode layer.

In the semiconductor device with the above structure, the first oxidesemiconductor layer and the second oxide semiconductor layer can beformed with use of the same mask, and the third oxide semiconductorlayer is formed to overlap with the second oxide semiconductor layer andhave a larger area than the second oxide semiconductor layer, so thatthe third oxide semiconductor layer covers the second oxidesemiconductor layer. In the semiconductor device with such a structure,the third oxide semiconductor layer is provided to cover and be incontact with the side surfaces of the first oxide semiconductor layerand the side surfaces of the second oxide semiconductor layer, wherebythe source electrode layer or the drain electrode layer provided overand in contact with the third oxide semiconductor layer is not incontact with the side surfaces of the second oxide semiconductor layer.Such a structure is preferable because generation of the leakage currentbetween the source electrode layer and the drain electrode layer(parasitic cannel) can be reduced.

Further, a structure of the oxide semiconductor stack is notparticularly limited as long as the source electrode layer or the drainelectrode layer is not in contact with the side surface of the secondoxide semiconductor layer. For example, the side surface of the firstoxide semiconductor layer may extend beyond the side surface of thesecond oxide semiconductor layer, so that the third oxide semiconductorlayer is in contact with part of a top surface of the first oxidesemiconductor layer.

In any of the above semiconductor devices, a region in the oxidesemiconductor stack, which does not overlap with the gate electrode,preferably contains dopant. In such a structure, the oxide semiconductorstack has a channel formation region which overlaps with the gateelectrode layer with the gate insulating film positioned therebetween,and a pair of low-resistance regions between which the channel formationregion is sandwiched in the channel length direction.

With an oxide semiconductor layer which includes low-resistance regionsbetween which a channel formation region is sandwiched in the channellength direction, the transistor has excellent on characteristics (e.g.,on-state current and field-effect mobility) and enables high-speedoperation and high-speed response. Further, the low-resistance regionsare formed in a self-aligned manner and do not overlap with the gateelectrode layer; thus a parasitic capacitor can be reduced. Reduction ofthe parasitic capacitor leads to lowering of power consumption of thewhole semiconductor device.

The concentration of the dopant in the low-resistance regions ispreferably higher than or equal to 5×10¹⁸/cm³ and lower than or equal to1×10²²/cm³.

The dopant may be added to the oxide semiconductor stack through thesource electrode layer or the drain electrode layer, depending on thethicknesses of the source electrode layer and the drain electrode layeror conditions for introducing the dopant. It is important that thedopant be prevented from being added to the channel formation region;thus, the thicknesses of the source electrode layer and the drainelectrode layer are smaller than the thickness of the gate electrodelayer.

In addition, any of the above semiconductor devices preferably furtherincludes: an interlayer insulating film which is provided over the gateelectrode layer and has a contact hole reaching the source electrodelayer or the drain electrode layer; and a wiring layer which is providedover the interlayer insulating film and connected to the sourceelectrode layer or the drain electrode layer through the contact hole.

According to one embodiment of the present invention, on characteristicsof the transistor (e.g., on-state current or field-effect mobility) canbe improved.

Further, according to one embodiment of the present invention, anormally-off transistor can be achieved. In addition, according to oneembodiment of the present invention, the threshold voltage of anormally-on transistor can be made to be close to zero.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a plan view of a semiconductor device according to oneembodiment of the present invention, FIGS. 1B and 1C are cross-sectionalviews thereof, and FIG. 1D is an energy band thereof.

FIGS. 2A to 2D illustrate a method for manufacturing a semiconductordevice according to one embodiment of the present invention.

FIG. 3 is a cross-sectional view illustrating a semiconductor deviceaccording to one embodiment of the present invention.

FIGS. 4A to 4E are cross-sectional views each illustrating asemiconductor device according to one embodiment of the presentinvention.

FIGS. 5A to 5C each illustrate a semiconductor device according to oneembodiment of the present invention.

FIGS. 6A and 6B each illustrate a semiconductor device according to oneembodiment of the present invention.

FIGS. 7A and 7B illustrate a semiconductor device according to oneembodiment of the present invention.

FIGS. 8A to 8D each illustrate an electronic device.

FIG. 9 is a graph showing the ionization potential.

FIG. 10 is an energy band diagram.

FIGS. 11A and 11B are a TEM image of Sample and a schematic diagramthereof.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that modes and details disclosedherein can be modified in various ways without departing from the spiritand the scope of the present invention. Therefore, the present inventionshould not be limited to the descriptions of the embodiments below.

Note that in structures of the present invention described below, likeportions or portions having similar functions are denoted by the samereference numerals in different drawings, and description thereof is notrepeated. Further, the same hatching pattern is applied to portionshaving similar functions, and the portions are not especially denoted byreference numerals in some cases.

Note that in each drawing described in this specification, the size, thefilm thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, embodiments of the present inventionare not limited to such scales.

Note that the ordinal numbers such as “first”, “second”, and the like inthis specification and the like are used for convenience and do notdenote the order of steps or the stacking order of layers. In addition,the ordinal numbers in this specification and the like do not denoteparticular names which specify the present invention.

Embodiment 1

In this embodiment, one embodiment of a semiconductor device and oneembodiment of a method for manufacturing the semiconductor device willbe described with reference to FIGS. 1A to 1D, FIGS. 2A to 2D, and FIG.3. In this embodiment, a transistor including an oxide semiconductorstack is shown as an example of the semiconductor device.

A transistor 510 illustrated in FIG. 1A, FIG. 1B, and FIG. 1C is anexample of a top-gate transistor. FIG. 1A is a top view, FIG. 1B is across-sectional view taken along chain line X-Y in FIG. 1A, and FIG. 1Cis a cross-sectional view taken along chain line V-W in FIG. 1A. Notethat in FIG. 1B and FIG. 1C, interfaces between oxide semiconductorlayers included in an oxide semiconductor stack 403 are schematicallydenoted by dotted lines. Depending on materials or deposition conditionsof the oxide semiconductor layers, the interfaces between the oxidesemiconductor layers are unclear in some cases. Further, in the casewhere the interfaces are unclear, a portion which can be called a mixedregion or a mixed layer including a plurality of oxide semiconductorlayers which are different from each other is formed in some cases.

As illustrated in a cross-sectional view in the channel length directionin FIG. 1B, the transistor 510 includes, over a substrate 400 which isprovided with an oxide insulating film 436 to have an insulatingsurface, the oxide semiconductor stack 403 including a first oxidesemiconductor layer, a second oxide semiconductor layer, and a thirdoxide semiconductor layer, a source electrode layer 405 a, a drainelectrode layer 405 b, a gate insulating film 402, and a gate electrodelayer 401. In the transistor 510, the first oxide semiconductor layer isformed over and in contact with the oxide insulating film 436, and thesecond oxide semiconductor layer is formed over the first oxidesemiconductor layer. Further, in the transistor 510, the oxidesemiconductor stack includes the third oxide semiconductor layer, andthe third oxide semiconductor layer is provided to cover side surfacesof the first oxide semiconductor layer and side surfaces of the secondoxide semiconductor layer. The edge portion of the third oxidesemiconductor layer is in contact with the oxide insulating film.

In the oxide semiconductor stack 403, a channel formation region whichoverlaps with the gate electrode layer 401 with the gate insulating film402 positioned therebetween includes three layers. In the channelformation region, a first channel formation region 121 c, a secondchannel formation region 122 c, and a third channel formation region 123c are stacked.

In the channel length direction, first low-resistance regions 121 a and121 b between which the first channel formation region 121 c issandwiched are provided. In the channel length direction, secondlow-resistance regions 122 a and 122 b between which the second channelformation region 122 c is sandwiched are provided. In the channel lengthdirection, third low-resistance regions 123 a and 123 b between whichthe third channel formation region 123 c is sandwiched are provided.

In addition, the oxide semiconductor stack 403 includes first regions121 d and 121 e, second regions 122 d and 122 e, and third regions 123 dand 123 e, which overlap with the source electrode layer 405 a or thedrain electrode layer 405 b.

In the oxide semiconductor stack 403 of the transistor 510 illustratedin FIGS. 1A to 1D, the first oxide semiconductor layer including thefirst low-resistance regions 121 a and 121 b, the first channelformation region 121 c, and the first regions 121 d and 121 e, thesecond oxide semiconductor layer including the second low-resistanceregions 122 a and 122 b, the second channel formation region 122 c, andthe second regions 122 d and 122 e, and the third oxide semiconductorlayer including the third low-resistance regions 123 a and 123 b, thethird channel formation region 123 c, and the third regions 123 d and123 e are stacked in this order.

In addition, in the transistor 510, the second oxide semiconductor layerhas a smaller energy gap than the first oxide semiconductor layer, andthe third oxide semiconductor layer has a larger energy gap than thesecond oxide semiconductor layer. Note that the energy gap of the firstoxide semiconductor layer is equivalent to that of the third oxidesemiconductor layer.

FIG. 1B is a cross-sectional view in the channel length direction, andin FIG. 1B, end portions of the second oxide semiconductor layer arecovered with end portions of the third oxide semiconductor layer (i.e.,side surfaces of the second regions 122 d and 122 e are covered with thethird regions 123 d and 123 e). With such a structure, generation of theleakage current between the source electrode layer 405 a and the drainelectrode layer 405 b (parasitic channel) can be reduced.

FIG. 1C is a cross-sectional view in the channel width direction, and asin FIG. 1B, the end portions of the second oxide semiconductor layer arepreferably covered with the end portions of the third oxidesemiconductor layer, (i.e., side surfaces of a second region 122 c arepreferably covered with a third region 123 c).

Further, FIG. 1D is an energy band diagram in the thickness direction(between D-D′) in FIG. 1B. In this embodiment, materials for a firstoxide semiconductor layer, a second oxide semiconductor layer, and athird oxide semiconductor layer are selected in order to satisfy theenergy band diagram of FIG. 1D. Note that when a buried channel isformed, sufficient effects can be obtained. Thus, an energy band of theoxide semiconductor stack is not necessarily limited to that shown inthe energy band diagram of FIG. 1D. That is, both conduction band andvalence band do not necessarily have a depression. For example, astructure such that only conduction band has a depression may be formed.

FIGS. 2A to 2D illustrate an example of a method for manufacturing atransistor.

First, over the substrate 400 having an insulating surface, the oxideinsulating film 436, a first oxide semiconductor layer 101, and a secondoxide semiconductor layer 102 are formed.

There is no particular limitation on a substrate that can be used as thesubstrate 400 having an insulating surface as long as it has heatresistance high enough to withstand heat treatment performed later. Forexample, a glass substrate of barium borosilicate glass,aluminoborosilicate glass, or the like, a ceramic substrate, a quartzsubstrate, or a sapphire substrate can be used. A single crystalsemiconductor substrate or a polycrystalline semiconductor substrate ofsilicon, silicon carbide, or the like; a compound semiconductorsubstrate of silicon germanium or the like; an SOI substrate; or thelike can be used as the substrate 400, or the substrate provided with asemiconductor element can be used as the substrate 400.

The semiconductor device may be manufactured using a flexible substrateas the substrate 400. In order to manufacture a flexible semiconductordevice, the transistor including the oxide semiconductor stack may bedirectly formed over a flexible substrate. Alternatively, the transistorincluding the oxide semiconductor stack may be formed over amanufacturing substrate, and then, the transistor may be separated andtransferred to a flexible substrate. Note that in order to separate thetransistor from the manufacturing substrate and transfer it to theflexible substrate, a separation layer may be provided between themanufacturing substrate and the transistor including the oxidesemiconductor stack.

The oxide insulating film 436 can be formed by a plasma CVD method, asputtering method, or the like, using silicon oxide, silicon oxynitride,aluminum oxide, aluminum oxynitride, hafnium oxide, gallium oxide,silicon nitride oxide, aluminum nitride oxide, or a mixed material ofany of these materials. The oxide insulating film 436 may have either asingle-layer structure or a stacked-layer structure. In this embodiment,a silicon oxide film formed by a sputtering method as the oxideinsulating film 436 is used.

In the transistor 510, the oxide insulating film 436 is in contact withthe lowermost layer and uppermost layer of the oxide semiconductorstack, and thus the film (bulk) of the oxide insulating film 436preferably contains at least oxygen in excess of stoichiometric ratio.For example, in the case where a silicon oxide film is used as the oxideinsulating film 436, the composition formula is SiO_(2+α) (α>0). Byusing such a film as the oxide insulating film 436, oxygen can besupplied to the oxide semiconductor stack formed above the oxideinsulating film 436, leading to favorable characteristics. By supply ofoxygen to the oxide semiconductor stack, oxygen vacancies in the filmcan be filled.

During a formation step of an oxide semiconductor stack over the oxideinsulating film 436, it is preferable that hydrogen or water becontained in the first oxide semiconductor layer 101 and the secondoxide semiconductor layer 102 as little as possible. Thus, aspretreatment of formation of the first oxide semiconductor layer 101 andthe second oxide semiconductor layer 102, it is preferable that thesubstrate provided with the oxide insulating film 436 be preheated in apreheating chamber of a sputtering apparatus, so that impurities such ashydrogen or moisture adsorbed on the substrate and the oxide insulatingfilm 436 are eliminated and evacuation is preferably performed. As anevacuation unit provided in the preheating chamber, a cryopump ispreferable.

Oxide semiconductors used for the oxide semiconductor stack preferablycontains at least indium (In) or zinc (Zn). In particular, In and Zn arepreferably contained. In addition, as a stabilizer for reducing thevariation in electric characteristics of a transistor using the oxide,the oxide semiconductor preferably contains gallium (Ga) in addition toIn and Zn. Tin (Sn) is preferably contained as a stabilizer. Hafnium(Hf) is preferably contained as a stabilizer. Aluminum (Al) ispreferably contained as a stabilizer.

As another stabilizer, one or plural kinds of lanthanoid such aslanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium(Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy),holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium(Lu) may be contained.

As the oxide semiconductor, for example, an indium oxide, a tin oxide, azinc oxide, a two-component metal oxide such as an In—Zn-based oxide, aSn—Zn-based oxide, an Al—Zn-based oxide, a Zn—Mg-based oxide, aSn—Mg-based oxide, an In—Mg-based oxide, or an In—Ga-based oxide, athree-component metal oxide such as an In—Ga—Zn-based oxide (alsoreferred to as IGZO), an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide,a Sn—Ga—Zn-based oxide, an Al—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide,an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-basedoxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, anIn—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide,an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-basedoxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, anIn—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide, a four-component metaloxide such as an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, anIn—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, anIn—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide can be used.

The oxide semiconductor may be either single crystal ornon-single-crystal. In the latter case, the oxide semiconductor may beeither amorphous or polycrystal. Further, the oxide semiconductor mayhave either an amorphous structure including a portion havingcrystallinity or a non-amorphous structure. Note that in some cases, theenergy gap is different between single crystal and non-single-crystaleven when a material of the single crystal is same as a material of thenon-single-crystal. Thus, it is important to select a crystal state of amaterial in the oxide semiconductor as appropriate. Materials of thefirst oxide semiconductor layer 101 and the second oxide semiconductorlayer 102 are selected so that the energy band diagram shown in FIG. 1Dis satisfied.

Further, as the oxide semiconductor stack, an oxide semiconductor filmincluding a crystal and having crystallinity (crystalline oxidesemiconductor film) can be used. A crystalline state in a crystallineoxide semiconductor film may be a state in which crystal axes areoriented in random directions or a state in which crystal axes areoriented in a certain direction.

For example, as the crystalline oxide semiconductor film, an oxidesemiconductor film including a crystal having a c-axis substantiallyperpendicular to a surface can be used.

The oxide semiconductor film including a crystal having a c-axissubstantially perpendicular to a surface has neither a single crystalstructure nor an amorphous structure and is a c-axis aligned crystallineoxide semiconductor (CAAC-OS) film having c-axis alignment.

The CAAC-OS film is not completely single crystal nor completelyamorphous. The CAAC-OS film is an oxide semiconductor film with acrystal-amorphous mixed phase structure where crystal parts andamorphous parts are included in an amorphous phase. Note that in mostcases, the crystal part fits inside a cube whose one side is less than100 nm. From an observation image obtained with a transmission electronmicroscope (TEM), a boundary between an amorphous part and a crystalpart in the CAAC-OS film is not clear. Further, with the TEM, a grainboundary in the CAAC-OS film is not found. Thus, in the CAAC-OS film, areduction in electron mobility, due to the grain boundary, issuppressed.

In each of the crystal parts included in the CAAC-OS film, a c-axis isaligned in a direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, triangular or hexagonal atomic arrangement which is seenfrom the direction perpendicular to the a-b plane is formed, and metalatoms are arranged in a layered manner or metal atoms and oxygen atomsare arranged in a layered manner when seen from the directionperpendicular to the c-axis. Note that, among crystal parts, thedirections of the a-axis and the b-axis of one crystal part may bedifferent from those of another crystal part. In this specification, asimple term “perpendicular” includes a range from 85° to 95°. Inaddition, a simple term “parallel” includes a range from −5° to 5°.

In the CAAC-OS film, distribution of crystal parts is not necessarilyuniform. For example, in the formation process of the CAAC-OS film, inthe case where crystal growth occurs from a surface side of the oxidesemiconductor film, the proportion of crystal parts in the vicinity ofthe surface of the oxide semiconductor film is higher than that in thevicinity of the surface where the oxide semiconductor film is formed insome cases. Further, when an impurity is added to the CAAC-OS film, thecrystal part in a region to which the impurity is added becomesamorphous in some cases.

Since the c-axes of the crystal parts included in the CAAC-OS film arealigned in the direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, the directions of the c-axes may be different from eachother depending on the shape of the CAAC-OS film (the cross-sectionalshape of the surface where the CAAC-OS film is formed or thecross-sectional shape of the surface of the CAAC-OS film). Note thatwhen the CAAC-OS film is formed, the direction of c-axis of the crystalpart is the direction parallel to a normal vector of the surface wherethe CAAC-OS film is formed or a normal vector of the surface of theCAAC-OS film. The crystal part is formed by film formation or byperforming treatment for crystallization such as heat treatment afterfilm formation.

With use of the CAAC-OS film in a transistor, change in electriccharacteristics of the transistor due to irradiation with visible lightor ultraviolet light can be reduced. Thus, the transistor has highreliability.

Each of the first oxide semiconductor layer 101 and the second oxidesemiconductor layer 102 has a thickness greater than or equal to 5 nmand less than or equal to 100 nm (preferably greater than or equal to 5nm and less than or equal to 30 nm) and can be formed by a sputteringmethod, a molecular beam epitaxy (MBE) method, a CVD method, a pulselaser deposition method, an atomic layer deposition (ALD) method, or thelike as appropriate. Each of the first oxide semiconductor layer 101 andthe second oxide semiconductor layer 102 may be formed with a sputteringapparatus where film formation is performed with surfaces of a pluralityof substrates set substantially perpendicular to a surface of asputtering target, which is a so-called columnar plasma (CP) sputteringsystem.

Note that each of the first oxide semiconductor layer 101 and the secondoxide semiconductor layer 102 is preferably deposited under conditionssuch that much oxygen is contained (for example, by a sputtering methodin an atmosphere of 100% oxygen) so as to be a film containing muchoxygen (preferably having a region containing an excess of oxygen ascompared to the stoichiometric composition ratio of the oxidesemiconductor in a crystalline state).

In this embodiment, the target used for formation of the first oxidesemiconductor layer 101 by a sputtering method is, for example, a metaloxide target having a composition ratio where In₂O₃:Ga₂O₃:ZnO=1:1:2[molar ratio], so that an In—Ga—Zn—O-based film is formed. The materialand the component of the target are not limited thereto, for example, ametal oxide target having a composition ratio, In₂O₃:Ga₂O₃:ZnO=1:1:1[molar ratio] may be used.

It is preferable to use a high-purity gas from which impurities such ashydrogen, water, a hydroxyl group, or a hydride are removed as asputtering gas used when the first oxide semiconductor layer 101 and thesecond oxide semiconductor layer 102 are formed.

The oxide insulating film 436 and the oxide semiconductor stack arepreferably formed in succession without being exposed to the air. Whenthe oxide insulating film 436 and the oxide semiconductor stack areformed in succession without being exposed to the air, impurities suchas hydrogen or moisture can be prevented from being adsorbed onto asurface of the oxide insulating film 436.

For example, the CAAC-OS film is formed by a sputtering method with apolycrystalline oxide semiconductor sputtering target. When ions collidewith the sputtering target, a crystal region included in the sputteringtarget may be separated from the target along an a-b plane; in otherwords, a sputtered particle having a plane parallel to an a-b plane(flat-plate-like sputtered particle or pellet-like sputtered particle)may flake off from the sputtering target. In that case, theflat-plate-like sputtered particle reaches a substrate while maintainingtheir crystal state, whereby the CAAC-OS film can be formed.

For the deposition of the CAAC-OS film, the following conditions arepreferably used.

By reducing the amount of impurities entering the CAAC-OS film duringthe deposition, the crystal state can be prevented from being broken bythe impurities. For example, the concentration of impurities (e.g.,hydrogen, water, carbon dioxide, or nitrogen) which exist in thedeposition chamber may be reduced. Furthermore, the concentration ofimpurities in a deposition gas may be reduced. Specifically, adeposition gas whose dew point is −80° C. or lower, preferably −100° C.or lower is used.

By increasing the substrate heating temperature during the deposition,migration of a sputtered particle is likely to occur after the sputteredparticle reaches a substrate surface. Specifically, the substrateheating temperature during the deposition is higher than or equal to100° C. and lower than or equal to 740° C., preferably higher than orequal to 200° C. and lower than or equal to 500° C. By increasing thesubstrate heating temperature during the deposition, when theflat-plate-like sputtered particle reaches the substrate, migrationoccurs on the substrate surface, so that a flat plane of theflat-plate-like sputtered particle is attached to the substrate.

Furthermore, it is preferable that the proportion of oxygen in thedeposition gas be increased and the power be optimized in order toreduce plasma damage at the deposition. The proportion of oxygen in thedeposition gas is 30 vol % or higher, preferably 100 vol %.

As an example of the sputtering target, an In—Ga—Zn—O compound target isdescribed below.

The In—Ga—Zn—O compound target, which is polycrystalline, is made bymixing InO_(x) powder, GaO_(y) powder, and ZnO_(z) powder in apredetermined molar ratio, applying pressure, and performing heattreatment at a temperature higher than or equal to 1000° C. and lowerthan or equal to 1500° C. Note that X, Y and Z are given positivenumbers. Here, the predetermined molar ratio of InO_(x) powder to GaOypowder and ZnO_(z) powder is, for example, 2:2:1, 8:4:3, 3:1:1, 1:1:1,4:2:3, or 3:1:2. The kinds of powder and the molar ratio for mixingpowder may be determined as appropriate depending on the desiredsputtering target.

In this embodiment, a first photolithography step is performed so thatthe formed oxide semiconductor stack is processed to have anisland-shaped first oxide semiconductor layer 101 and an island shapedsecond oxide semiconductor layer 102, as illustrated in FIG. 2A. Aresist mask used for forming the island-shaped first oxide semiconductorlayer 101 and the island-shaped second oxide semiconductor layer 102 maybe formed by an inkjet method. Formation of the resist mask by an inkjetmethod needs no photomask; thus, manufacturing cost can be reduced.

Note that the etching of the oxide semiconductor stack may be performedusing either dry etching or wet etching or both of them. As an etchantused for wet etching of the oxide semiconductor film, for example, amixed solution of phosphoric acid, acetic acid, and nitric acid, or thelike can be used. In addition, ITO07N (produced by KANTO CHEMICAL CO.,INC.) may also be used.

Next, a third oxide semiconductor layer 103 is formed to cover theisland-shaped first oxide semiconductor layer 101 and the island-shapedsecond oxide semiconductor layer 102. As a result, the oxidesemiconductor stack 403 is formed. The third oxide semiconductor layer103 is formed using a target same as that for the first oxidesemiconductor layer 101. The deposition conditions of the third oxidesemiconductor layer 103 are the same as that of the first oxidesemiconductor layer 101; thus description thereof is omitted here. Notethat by a second photolithography step, the third oxide semiconductorlayer 103 which overlaps with the second oxide semiconductor layer 102and has a top surface shape larger than a plan area of the second oxidesemiconductor layer 102 is formed.

Then, heat treatment may be performed on the oxide semiconductor stack403 in order to remove excess hydrogen (including water and a hydroxylgroup) (to perform dehydration or dehydrogenation). The temperature ofthe heat treatment is higher than or equal to 300° C. and lower than orequal to 700° C., or lower than the strain point of the substrate. Theheat treatment can be performed under reduced pressure or in a nitrogenatmosphere, or the like. For example, the substrate is put in anelectric furnace which is a kind of heat treatment apparatus, and theoxide semiconductor stack 403 is subjected to the heat treatment at 450°C. for an hour in a nitrogen atmosphere.

Further, a heat treatment apparatus used is not limited to an electricfurnace, and a device for heating a process object by heat conduction orheat radiation from a heating element such as a resistance heatingelement may be alternatively used. For example, an RTA (rapid thermalanneal) apparatus such as a GRTA (gas rapid thermal anneal) apparatus oran LRTA (lamp rapid thermal anneal) apparatus can be used. An LRTAapparatus is an apparatus for heating an object to be processed byradiation of light (an electromagnetic wave) emitted from a lamp such asa halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high pressure sodium lamp, or a high pressure mercury lamp. AGRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas. As the high-temperature gas, an inert gas whichdoes not react with an object to be processed by heat treatment, such asnitrogen or a rare gas like argon, is used.

For example, as the heat treatment, GRTA may be performed as follows.The substrate is put in an inert gas heated at high temperature of 650°C. to 700° C., is heated for several minutes, and is taken out from theinert gas.

Note that in the heat treatment, it is preferable that water, hydrogen,and the like be not contained in nitrogen or a rare gas such as helium,neon, or argon. The purity of nitrogen or the rare gas such as helium,neon, or argon which is introduced into the heat treatment apparatus isset to preferably 6N (99.9999%) or higher, far preferably 7N (99.99999%)or higher (that is, the impurity concentration is preferably 1 ppm orlower, far preferably 0.1 ppm or lower).

In addition, after the oxide semiconductor stack 403 is heated by theheat treatment, a high-purity oxygen gas, a high-purity N₂O gas, orultra dry air (the moisture amount is less than or equal to 20 ppm (−55°C. by conversion into a dew point), preferably less than or equal to 1ppm, far preferably less than or equal to 10 ppb, in the measurementwith use of a dew point meter of a cavity ring down laser spectroscopy(CRDS) system) may be introduced into the same furnace while the heatingtemperature is maintained or slow cooling is performed to lower thetemperature from the heating temperature. It is preferable that water,hydrogen, or the like be not contained in the oxygen gas or the N₂O gas.The purity of the oxygen gas or the N₂O gas which is introduced into theheat treatment apparatus is preferably 6N or more, far preferably 7N ormore (i.e., the impurity concentration in the oxygen gas or the N₂O gasis preferably 1 ppm or lower, far preferably 0.1 ppm or lower). Theoxygen gas or the N₂O gas acts to supply oxygen that is a main componentof the oxide semiconductor and that has been reduced by removing animpurity for the dehydration or dehydrogenation, so that the oxidesemiconductor stack 403 can have high purity and be an electricallyi-type (intrinsic) oxide semiconductor film.

Next, a conductive film to be the source electrode layer and the drainelectrode layer (including a wiring formed from the same layer as thesource electrode layer and the drain electrode layer) is formed over theoxide semiconductor stack 403. The conductive film is formed using amaterial that can withstand heat treatment in a later step. As theconductive film used for the source electrode layer and the drainelectrode layer, for example, a metal film containing an elementselected from Al, Cr, Cu, Ta, Ti, Mo, or W, a metal nitride filmcontaining any of the above elements as its component (a titaniumnitride film, a molybdenum nitride film, or a tungsten nitride film), orthe like can be used. A metal film having a high melting point, whichincludes Ti, Mo, W, or the like, or a metal nitride film of any of theseelements (a titanium nitride film, a molybdenum nitride film, and atungsten nitride film) may be stacked on one of or both of a lower sideor an upper side of a metal film including Al, Cu, or the like.Alternatively, the conductive film used for the source electrode layerand the drain electrode layer may be formed using a conductive metaloxide. Indium oxide (In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO), indiumoxide-tin oxide (In₂O₃—SnO₂), indium oxide-zinc oxide (In₂O₃—ZnO), orany of these metal oxide materials containing silicon oxide can be usedas the conductive metal oxide.

A resist mask is formed over the conductive film by a thirdphotolithography step. Etching is selectively performed, so that thesource electrode layer 405 a and the drain electrode layer 405 b areformed. Then, the resist mask is removed. FIG. 2B illustrates a crosssection at this stage. In this embodiment, a 10-nm-thick tungsten filmis formed as the source electrode layer 405 a and the drain electrodelayer 405 b. When the source electrode layer 405 a and the drainelectrode layer 405 b have the small thickness as the above, coveragewith the gate insulating film 402 formed thereover can be favorable, andthrough the source electrode layer 405 a and the drain electrode layer405 b, dopant can be introduced in the oxide semiconductor stack 403below the source electrode layer 405 a and the drain electrode layer 405b.

Next, the gate insulating film 402 is formed to cover the oxidesemiconductor stack 403, the source electrode layer 405 a, and the drainelectrode layer 405 b.

The gate insulating film 402 can be formed to have a thickness greaterthan or equal to 1 nm and less than or equal to 20 nm by a sputteringmethod, an MBE method, a CVD method, a pulse laser deposition method, anALD method, or the like as appropriate. Alternatively, the gateinsulating film 402 may be formed with a sputtering apparatus where filmformation is performed with surfaces of a plurality of substrates setsubstantially perpendicular to a surface of a sputtering target, whichis a so-called columnar plasma (CP) sputtering system.

The gate insulating film 402 can be formed using a silicon oxide film, agallium oxide film, an aluminum oxide film, a silicon nitride film, asilicon oxynitride film, an aluminum oxynitride film, or a siliconnitride oxide film.

The gate insulating film 402 can be formed using a high-k material suchas hafnium oxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y) (x>0,y>0)), hafnium silicate to which nitrogen is added (HfSiO_(x)N_(y) (x>0,y>0)), hafnium aluminate (HfAl_(x)O_(y) (x>0, y>0)), or lanthanum oxide,whereby gate leakage current can be reduced. Further, the gateinsulating film 402 may have either a single-layer structure or astacked-layer structure.

Next, as illustrated in FIG. 2C, oxygen 431 is introduced into the oxidesemiconductor stack 403 with use of the source electrode layer 405 a andthe drain electrode layer 405 b as masks. By introducing oxygen(including at least one of an oxygen radical, an oxygen atom, and anoxygen ion), oxygen is supplied at least into the third oxidesemiconductor layer. As a method for introducing oxygen, an ionimplantation method, an ion doping method, a plasma immersion ionimplantation method, plasma treatment, or the like may be used.

By introducing oxygen into the oxide semiconductor stack 403, a regionwhich is in the oxide semiconductor stack 403 and overlaps with neitherthe source electrode layer 405 a nor the drain electrode layer 405 b hasa higher oxygen concentration than a region which overlaps with thesource electrode layer 405 a or the drain electrode layer 405 b. It ispreferable that the oxygen content in the region which overlaps withneither the source electrode layer 405 a nor the drain electrode layer405 b be increased to higher than that of the stoichiometric compositionratio by oxygen introducing treatment. For example, a peak of theconcentration of oxygen which is introduced into the region of the oxidesemiconductor stack 403 by the oxygen introducing treatment ispreferably higher than or equal to 1×10¹⁸/cm³ and lower than or equal to5×10²¹/cm³.

In this embodiment, as a region which contains oxygen in excess ofstoichiometry (oxygen-excess region), a first oxygen-excess region 111,a second oxygen-excess region 112, and a third oxygen-excess region 113are formed in a self-aligned manner, by introducing the oxygen 431. Notethat the oxygen 431 may be contained at least in the interface betweenthe third oxide semiconductor layer 103 and the gate insulating film402. Thus, depending on the depth at which the oxygen 431 is introduced,in the first oxide semiconductor layer 101 and the second oxidesemiconductor layer 102, the oxygen concentration in a region whichoverlaps with the source electrode layer 405 a or the drain electrodelayer 405 b are equal to that in a region which overlaps with neitherthe source electrode layer 405 a nor the drain electrode layer 405 b, insome cases. The depth at which the oxygen is introduced into the oxidesemiconductor stack 403 may be controlled by appropriately setting animplantation condition such as acceleration voltage or a dose, or thethickness of the gate insulating film 402 through which the oxygenpasses.

The timing of introducing the oxygen 431 is not limited to the timingafter formation of the gate insulating film 402. However, when oxygen isintroduced through a film stacked over the oxide semiconductor stack403, the depth at which oxygen is introduced (introduction region) iscontrolled easily, which enables efficient oxygen implantation into theoxide semiconductor stack 403.

Further, heat treatment may be performed after the oxygen 431 isintroduced. As the preferable heating conditions, the heat temperatureis higher than or equal to 250° C. and lower than or equal to 700° C.,preferably higher than or equal to 300° C. and lower than or equal to450° C., and the heat treatment is preferably performed in an oxygenatmosphere. Further, the heat treatment may be performed in a nitrogenatmosphere, or under reduced pressure or the air (ultra dry air).

In the case where at least one of layers in the oxide semiconductorstack is a crystalline oxide semiconductor film, part of the crystallineoxide semiconductor film becomes amorphous in some cases by introductionof the oxygen 431. In that case, the crystallinity of the oxidesemiconductor stack can be recovered by performing heat treatmentthereon after the introduction of the oxygen 431.

Further, in the case where an oxygen-excess region is formed in theoxide semiconductor stack 403, oxygen vacancies can be immediatelyfilled; thus, charge trapping centers in the oxide semiconductor stack403 can be reduced. There is an oxygen vacancy in a portion where oxygenis detached in the oxide semiconductor stack 403 and a donor level whichleads to variation in the electric characteristics of a transistor isformed owing to the oxygen vacancy. Since oxygen vacancies in the filmcan be filled by introduction of oxygen, with use of such an oxidesemiconductor stack for transistors, variation in threshold voltagesV_(th) of the transistors owing to oxygen vacancies, and the shift ofthe threshold voltage ΔV_(th) can be reduced. In addition, the thresholdvoltage can be positively shifted to make a normally-off transistor.

Next, the gate electrode layer 401 is formed over the gate insulatingfilm 402 by a plasma CVD method, a sputtering method, or the like. Thegate electrode layer 401 can be formed using a metal material such asmolybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium,neodymium, or scandium or an alloy material which contains any of thesematerials as its main component. Alternatively, a semiconductor filmtypified by a polycrystalline silicon film doped with an impurityelement such as phosphorus, or a silicide film such as a nickel silicidefilm may be used as the gate electrode layer 401. The gate electrodelayer 401 may have a single-layer structure or a stacked-layerstructure.

The gate electrode layer 401 can also be formed using a conductivematerial such as indium tin oxide, indium oxide containing tungstenoxide, indium zinc oxide containing tungsten oxide, indium oxidecontaining titanium oxide, indium tin oxide containing titanium oxide,indium zinc oxide, or indium tin oxide to which silicon oxide is added.It is also possible that the gate electrode layer 401 has a stackedstructure of the above conductive material and the above metal material.

As one layer of the gate electrode layer 401 which is in contact withthe gate insulating film 402, a metal oxide containing nitrogen,specifically, an In—Ga—Zn-based oxide film containing nitrogen, anIn—Sn-based oxide film containing nitrogen, an In—Ga-based oxide filmcontaining nitrogen, an In—Zn-based oxide film containing nitrogen, aSn-based oxide film containing nitrogen, an In-based oxide filmcontaining nitrogen, or a metal nitride (e.g., InN or SnN) film can beused. Such a film has a work function of 5 eV or higher, preferably 5.5eV or higher, and use of this film as the gate electrode layer enablesthe threshold voltage of electric characteristics of a transistor makesa positive value. Accordingly, a so-called normally-off switchingelement can be obtained.

Next, treatment in which dopant 421 is selectively introduced ispreferably performed. In this treatment, with use of the gate electrodelayer 401 as a mask, the dopant 421 is introduced though the gateinsulating film 402, so that the first low-resistance regions 121 a and121 b, the second low-resistance regions 122 a and 122 b, and the thirdlow-resistance regions 123 a and 123 b are formed. Through thistreatment, the first low-resistance regions 121 a and 121 b betweenwhich the first channel formation region 121 c is sandwiched are formedin the channel length direction in a self-aligned manner. Further, thesecond low-resistance regions 122 a and 122 b between which the secondchannel formation region 122 c is sandwiched are formed in the channellength direction in a self-aligned manner. Furthermore, the thirdlow-resistance regions 123 a and 123 b between which the third channelformation region 123 c is sandwiched are formed in the channel lengthdirection in a self-aligned manner.

In the transistor 510 of this embodiment, the first low-resistanceregions 121 a and 121 b, the second low-resistance regions 122 a and 122b, and the third low-resistance regions 123 a and 123 b are regionscontaining the dopant and excess oxygen.

In addition, in this treatment, the dopant 421 is selectively introducedinto the oxide semiconductor stack 403 (at least into the third oxidesemiconductor layer 103) through the gate insulating film 402, thesource electrode layer 405 a, and the drain electrode layer 405 b, sothat the first regions 121 d and 121 e, the second regions 122 d and 122e, and the third regions 123 d and 123 e are formed (see FIG. 2D). Thedopant 421 is also introduced below the source electrode layer 405 a andthe drain electrode layer 405 b, whereby resistance of the first regions121 d and 121 e, the second regions 122 d and 122 e, and the thirdregions 123 d and 123 e can be reduced.

The low-resistance regions between which the channel formation region issandwiched is formed in the channel length direction in the oxidesemiconductor stack 403 by introducing the dopant 421, whereby the oncharacteristics of the transistor 510 are improved, high-speed operationand high-speed response of the transistor can be achieved. Further, thelow-resistance regions are formed in a self-aligned manner and do notoverlap with the gate electrode; thus, parasitic capacitor can bereduced. A reduction in parasitic capacitor leads to lowering of powerconsumption of the whole semiconductor device.

In this embodiment, since the source electrode layer 405 a and the drainelectrode layer 405 b are thin, the dopant 421 is also introduced intoportions of the oxide semiconductor stack 403 below the source electrodelayer 405 a and the drain electrode layer 405 b. Depending on thethicknesses of the source electrode layer 405 a and the drain electrodelayer 405 b and the conditions for introducing the dopant 421, thedopant 421 is not introduced into portions of the oxide semiconductorstack below the source electrode layer 405 a and the drain electrodelayer 405 b.

The introduction of the dopant 421 may be controlled by appropriatelysetting an implantation condition such as acceleration voltage or adose, or the thickness of the gate insulating film 402 through which thedopant passes. For example, in the case where boron is used and boronions are implanted by an ion implantation method, the dose can begreater than or equal to 1×10¹³ ions/cm² and less than or equal to5×10¹⁶ ions/cm².

The concentrations of the dopant 421 in the first low-resistance regions121 a and 121 b, the second low-resistance regions 122 a and 122 b, andthe third low-resistance regions 123 a and 123 b are preferably higherthan or equal to 5×10¹⁸/cm³ and lower than or equal to 1×10²²/cm³.

The introduction of the dopant may be performed while the substrate 400is heated.

Note that the treatment in which the dopant 421 is introduced into thefirst low-resistance regions 121 a and 121 b, the second low-resistanceregions 122 a and 122 b, and the third low-resistance regions 123 a and123 b may be performed plural times, and a plural kinds of dopants maybe used.

After the dopant 421 is introduced, heat treatment may be performed. Asthe preferable heating conditions, the heat temperature is higher thanor equal to 300° C. and lower than or equal to 700° C., preferablyhigher than or equal to 300° C. and lower than or equal to 450° C., andthe heat treatment is preferably performed for an hour in an oxygenatmosphere. Further, the heat treatment may be performed in a nitrogenatmosphere, or under reduced pressure or air (ultra dry air).

In the case where at least one of layers in the oxide semiconductorstack is a crystalline oxide semiconductor film, part of the crystallineoxide semiconductor film becomes amorphous in some cases by introductionof the dopant 421. In that case, the crystallinity of the oxidesemiconductor stack can be recovered by performing heat treatmentthereon after the introduction of the dopant 421.

In this embodiment, boron is used as the dopant. Thus, the firstlow-resistance regions 121 a and 121 b, the second low-resistanceregions 122 a and 122 b, and the third low-resistance regions 123 a and123 b contain boron and an excess of oxygen.

Through the above-described steps, the transistor 510 of this embodimentis formed.

Note that an insulating film 407 may be formed to cover the transistor(see FIG. 1B and FIG. 1C).

As the insulating film 407, a silicon oxide film, or an inorganicinsulating film such as, typically, an aluminum oxide film, a siliconoxynitride film, an aluminum oxynitride film, or a gallium oxide filmcan be used. For example, as the insulating film 407, a stacked layerincluding a silicon oxide film and an aluminum oxide film can be used.

An aluminum oxide film which can be used for the insulating film 407 hasa superior shielding effect (blocking effect), which is not permeable toeither oxygen or impurities such as hydrogen or moisture.

As the insulating film 407, a planarization insulating film may be used.As the planarization insulating film, an organic material such as apolyimide resin, an acrylic resin, or a benzocyclobutene-based resin canbe used. Other than such organic materials, it is also possible to use alow-dielectric constant material (a low-k material) or the like. Notethat the planarization insulating film may be formed by stacking aplurality of insulating films formed from these materials.

Next, opening portions reaching the source electrode layer 405 a and thedrain electrode layer 405 b are formed in the insulating film 407, and awiring layer electrically connected to the source electrode layer 405 aor the drain electrode layer 405 b is formed in the opening portions.With use of this wiring layer, the transistor 510 is connected toanother transistor, which can lead to formation of a variety ofcircuits.

The technical feature of the transistor described in this embodiment isto include an oxide semiconductor stack in which oxide semiconductorswith different energy gaps are stacked, and to have, in the oxidesemiconductor stack, a region which contains oxygen in excess of itsstoichiometry. Thus, introduction of an impurity into the oxidesemiconductor stack 403 is not necessarily performed. For example, inthe manufacturing process illustrated in FIGS. 2A to 2D, after the gateelectrode layer 401 is formed, an impurity is not introduced, and theinsulating film 407 is formed over the gate insulating film 402 and thegate electrode layer 401. As a result, a transistor 520 illustrated inFIG. 3 can be formed.

The transistor 520 illustrated in FIG. 3 includes, over the substrate400 which is provided with the oxide insulating film 436 to have aninsulating surface, the oxide semiconductor stack 403 including a firstoxide semiconductor layer, a second oxide semiconductor layer, and athird oxide semiconductor layer, the source electrode layer 405 a, thedrain electrode layer 405 b, the gate insulating film 402, and the gateelectrode layer 401 overlapping with the oxide semiconductor stack 403with the gate insulating film 402 positioned therebetween.

In the transistor 520, the oxide semiconductor stack 403 has: a firstoxygen-excess region 111, a second oxygen-excess region 112, and a thirdoxygen-excess region 113, which are formed in a self-aligned manner withuse of the source electrode layer 405 a and the drain electrode layer405 b as masks; and first regions 131 d and 131 e, second regions 132 dand 132 e, and third regions 133 d and 133 e, which overlap with thesource electrode layer 405 a or the drain electrode layer 405 b.

The portions which overlap with neither the source electrode layer 405 anor the drain electrode layer 405 b (the first oxygen-excess region 111,the second oxygen-excess region 112, and the third oxygen-excess region113) have a higher oxygen concentration than the portions which overlapwith the source electrode layer 405 a and the drain electrode layer 405b (the first regions 131 d and 131 e, the second regions 132 d and 132e, and the third regions 133 d and 133 e). However, composition elementsin the both portions are same.

The oxide semiconductor stack 403 used in the transistor 510 or thetransistor 520 is highly purified, and oxygen vacancies therein arefilled; impurities such as hydrogen or water therein are sufficientlyremoved. The hydrogen concentration in the oxide semiconductor stack 403is lower than or equal to 5×10¹⁹/cm³, preferably lower than or equal to5×10¹⁸/cm³. Note that the concentration of hydrogen in the oxidesemiconductor stack 403 is measured by secondary ion mass spectrometry(SIMS).

In the case of the transistor using the highly purified oxidesemiconductor stack 403 containing an excess of oxygen that fills oxygenvacancies according to this embodiment, the current value in the offstate (off-state current value) can be reduced to a value less than orequal to 100 zA per micrometer of channel width at room temperature (1zA (zeptoampere)=1×10⁻²¹ A), preferably less than or equal to 10 zA/μm,further preferably less than or equal to 1 zA/μm, still furtherpreferably less than or equal to 100 yA/μm.

Further, in each of the transistor 510 and the transistor 520 describedin this embodiment, the third oxide semiconductor layer is formed tocover the side surfaces of the first oxide semiconductor layer and sidesurfaces of the second oxide semiconductor layer. With such a structure,an increase in oxygen vacancies in the second oxide semiconductor layeris suppressed, and the threshold voltage of the transistor can be closeto zero. Furthermore, the second oxide semiconductor layer serves as aburied channel, which results in a decrease in carrier scattering. As aresult, high field-effect mobility can be achieved.

With a structure in which the second oxide semiconductor layer with asmall energy gap is sandwiched between the first oxide semiconductorlayer and the third oxide semiconductor layer with large energy gaps,the off-state current (leakage current) of the transistor can bereduced.

With use of the thus obtained transistor having excellent electriccharacteristics, a semiconductor device with high performance and highreliability can be provided.

The methods and structures described in this embodiment can be combinedas appropriate with any of the methods and structures described in theother embodiments.

Embodiment 2

In this embodiment, examples of transistors formed by the manufacturingprocess, part of which is changed, described in Embodiment 1 will bedescribed with reference to FIGS. 4A to 4E. The transistors described inthis embodiment are just partly different from those in Embodiment 1;thus the same reference numerals are used to denote the same portionsfor simplification, and detailed description of the same portion isomitted in this embodiment.

A transistor 530 illustrated in FIG. 4A has a structure in which part ofthe oxide insulating film 436 is etched to be thin with use of a maskwhich is used for processing the first oxide semiconductor layer and thesecond oxide semiconductor layer to have an island shape (or with use ofthe first island-shaped oxide semiconductor layer and the secondisland-shaped oxide semiconductor layer which are manufactured by theprocessing, as a mask). In the transistor 530, a region in the oxideinsulating film 436, which overlaps with the first and secondisland-shaped oxide semiconductor layers, has a larger thickness thanthe other region in the oxide insulating film 436, which does notoverlap with the first and second island-shaped oxide semiconductorlayers. When part of the oxide insulating film 436 is etched at the timeof processing the first oxide semiconductor layer and the second oxidesemiconductor layer to have an island shape, an etching residue of thefirst oxide semiconductor layer or the like is removed, so thatgeneration of leakage current can be reduced.

A transistor 540 illustrated in FIG. 4B has a structure in which theoxide semiconductor stack 403 is formed by performing photolithographythree times. The oxide semiconductor stack 403 included in thetransistor 540 is formed as follows: after a first oxide semiconductorlayer is formed, the first oxide semiconductor layer is processed tohave an island shape with use of a first mask; a second oxidesemiconductor layer is formed over the first island-shaped oxidesemiconductor layer; the second oxide semiconductor layer is processedto have an island shape with use of a second mask; a third oxidesemiconductor layer is formed over the first and second island-shapedoxide semiconductor layers; and the third oxide semiconductor layer isprocessed to have an island shape with use of a third mask.

Note that the transistor 540 has a structure in which a side surface ofthe first oxide semiconductor layer extends beyond a side surface of thesecond oxide semiconductor layer, and the third oxide semiconductorlayer is in contact with part of a top surface of the first oxidesemiconductor layer. The third regions 123 d and 123 e corresponding toend portions of the third oxide semiconductor layer are in contact withand overlap with the first regions 121 d and 121 e corresponding to endportions of the first oxide semiconductor layer.

A transistor 550 illustrated in FIG. 4C has a structure in which asource electrode layer has a stacked structure of a source electrodelayer 405 c and the source electrode layer 405 a, a drain electrodelayer has a stacked structure of a drain electrode layer 405 d and thedrain electrode layer 405 b, and a wiring layer 465 a and a wiring layer465 b are formed to reach the source electrode layer 405 c and the drainelectrode layer 405 d which are lower layers of the source electrodelayer and the drain electrode layer. By an etching step for formingcontact holes in the insulating film 407, part of the source electrodelayer 405 a or the drain electrode layer 405 b is over-etched andremoved in some cases. In the transistor 550, the source electrode layerand the drain electrode layer each have a stacked layer structure, and aconductive layer which is a lower layer can serve as an etching stopper.

In the transistor 550 in this embodiment, as the source electrode layer405 c and the drain electrode layer 405 d which are lower layers, atungsten film or a tantalum nitride film is used, and as the sourceelectrode layer 405 a and the drain electrode layer 405 b which areupper layers thicker than the lower layers, a copper film or an aluminumfilm is used. The thicknesses of the source electrode layer 405 a andthe drain electrode layer 405 b in FIG. 4C are greater than or equal to5 nm and less than or equal to 15 nm, whereby coverage with the gateinsulating film 402 formed thereover can be favorable. Note that in thisembodiment, the wiring layer 465 a and the wiring layer 465 b each havea stacked structure of a tantalum nitride film and a copper film or astacked structure of a tantalum nitride film and a tungsten film,whereby the contact resistance can be achieved.

A transistor 560 illustrated in FIG. 4D has a structure in which a firstoxide semiconductor layer, a second oxide semiconductor layer, and athird oxide semiconductor layer are processed by performing aphotolithography step once with use of one mask, so that the oxidesemiconductor stack 403 is formed. In the oxide semiconductor stack 403included in the transistor 560, an end portion of the first oxidesemiconductor layer, an end portion of the second oxide semiconductorlayer, and an end portion of the third oxide semiconductor layer arealigned; the three oxide semiconductor layers have the same shape. Inother words, in the oxide semiconductor stack 403, side surfaces (endportions) of the first oxide semiconductor layer and side surfaces (endportions) of the second oxide semiconductor layer are exposed.

Formation of the oxide semiconductor stack 403 by performing aphotolithography step once makes it possible to reduce the number ofsteps, and thus, manufacturing cost of a semiconductor device can bereduced. Note that in the transistor 560, when the source electrodelayer 405 a and the drain electrode layer 405 b are formed over thethird oxide semiconductor layer to be in contact with only a top surfaceof the third oxide semiconductor layer or to be in contact with only atop surface and a side surface of the third oxide semiconductor layer, astructure in which the source electrode layer 405 a and the drainelectrode layer 405 b are not in contact with the side surface of thesecond oxide semiconductor layer can be formed. Such a structure ispreferable because generation of leakage current between the sourceelectrode layer and the drain electrode layer (parasitic channel) of thetransistor can be reduced.

The oxide semiconductor stack 403 does not necessarily have athree-layer structure. For example, a transistor 570 illustrated in FIG.4E includes the oxide semiconductor stack 403 with a two-layer structureof a first oxide semiconductor layer and a second oxide semiconductorlayer. In the transistor 570, the second oxide semiconductor layeroverlaps with the first oxide semiconductor layer and has a larger areathan the first oxide semiconductor layer, thereby being formed to coverthe first oxide semiconductor layer. With such a structure, an increaseof oxygen vacancies in the first oxide semiconductor layer can besuppressed, and the threshold voltage of the transistor can be close tozero. Note that in the transistor 570, an aluminum oxide film is formedas the oxide insulating film 436, whereby oxygen can be prevented frombeing released to an insulating film in contact with the first oxidesemiconductor layer, which is preferable.

In the transistor 570 illustrated in FIG. 4E, the source electrode layer405 a or the drain electrode layer 405 b formed over and in contact withthe second oxide semiconductor layer is not in contact with the sidesurfaces of the first oxide semiconductor layer. Such a structure ispreferable because generation of leakage current between the sourceelectrode layer 405 a and the drain electrode layer 405 b (parasiticchannel) can be reduced.

The methods and structures described in this embodiment can be combinedas appropriate with any of the methods and structures described in theother embodiments.

Embodiment 3

A semiconductor device having a display function (also referred to as adisplay device) can be manufactured using any of the transistorsdescribed in Embodiment 1 or Embodiment 2. Moreover, some or all of thedriver circuits which include the transistors can be formed over asubstrate where the pixel portion is formed, whereby a system-on-panelcan be obtained.

In FIG. 5A, a sealant 4005 is provided so as to surround a pixel portion4002 provided over a first substrate 4001, and the pixel portion 4002 issealed between the first substrate 4001 and the second substrate 4006.In FIG. 5A, a signal line driver circuit 4003 and a scan line drivercircuit 4004 which are formed using a single crystal semiconductor filmor a polycrystalline semiconductor film over a substrate separatelyprepared are mounted in a region that is different from the regionsurrounded by the sealant 4005 over the first substrate 4001. Varioussignals and potential are supplied to the signal line driver circuit4003 and the scan line driver circuit 4004 each of which is separatelyformed, and the pixel portion 4002 from flexible printed circuits (FPCs)4018 a and 4018 b.

In FIGS. 5B and 5C, the sealant 4005 is provided so as to surround thepixel portion 4002 and the scan line driver circuit 4004 which areprovided over the first substrate 4001. The second substrate 4006 isprovided over the pixel portion 4002 and the scan line driver circuit4004. Consequently, the pixel portion 4002 and the scan line drivercircuit 4004 are sealed together with the display element, by the firstsubstrate 4001, the sealant 4005, and the second substrate 4006. InFIGS. 5B and 5C, the signal line driver circuit 4003 which is formedusing a single crystal semiconductor film or a polycrystallinesemiconductor film over a substrate separately prepared is mounted in aregion that is different from the region surrounded by the sealant 4005over the first substrate 4001. In FIGS. 5B and 5C, various signals andpotential are supplied to the signal line driver circuit 4003 which isseparately formed, the scan line driver circuit 4004, and the pixelportion 4002 from an FPC 4018.

Although FIGS. 5B and 5C each illustrate an example in which the signalline driver circuit 4003 is formed separately and mounted on the firstsubstrate 4001, the display device according to the present invention isnot limited to this structure. The scan line driver circuit may beformed separately and then mounted, or only part of the signal linedriver circuit or part of the scan line driver circuit may be formedseparately and then mounted.

Note that a connection method of a separately formed driver circuit isnot particularly limited, and a chip on glass (COG) method, a wirebonding method, a tape automated bonding (TAB) method or the like can beused. FIG. 5A illustrates an example in which the signal line drivercircuit 4003 and the scan line driver circuit 4004 are mounted by a COGmethod. FIG. 5B illustrates an example in which the signal line drivercircuit 4003 is mounted by a COG method. FIG. 5C illustrates an examplein which the signal line driver circuit 4003 is mounted by a TAB method.

Further, a display device includes a panel in which a display element issealed, and a module in which an IC or the like including a controlleris mounted to the panel.

Note that a display device in this specification means an image displaydevice, a display device, or a light source (including a lightingdevice). Furthermore, the display device also includes the followingmodules in its category: a module to which a connector such as an FPC, aTAB tape, or a TCP is attached; a module having a TAB tape or a TCP atthe tip of which a printed wiring board is provided; and a module inwhich an integrated circuit (IC) is directly mounted on a displayelement by a COG method.

The pixel portion and the scan line driver circuit provided over thefirst substrate include a plurality of transistors, and any of thetransistors which is described in Embodiment 1 or Embodiment 2 can beapplied thereto.

As the display element provided in the display device, a liquid crystalelement (also referred to as a liquid crystal display element) or alight-emitting element (also referred to as a light-emitting displayelement) can be used. The light-emitting element includes, in itscategory, an element whose luminance is controlled by a current or avoltage, and specifically includes, in its category, an inorganicelectroluminescent (EL) element, an organic EL element, and the like.Furthermore, a display medium whose contrast is changed by an electriceffect, such as electronic ink, can be used.

Embodiments of the semiconductor device will be described with referenceto FIGS. 5A to 5C and FIGS. 6A and 6B. FIGS. 6A and 6B arecross-sectional views along line M-N in FIG. 5B.

As illustrated in FIGS. 6A and 6B, the semiconductor device includes aconnection terminal electrode 4015 and a terminal electrode 4016, andthe connection terminal electrode 4015 and the terminal electrode 4016are electrically connected to a terminal included in the FPC 4018through an anisotropic conductive film 4019.

The connection terminal electrode 4015 is formed using the sameconductive film as a first electrode layer 4030, and the terminalelectrode 4016 is formed using the same conductive film as source anddrain electrode layers of transistors 4040, 4010, and 4011.

The pixel portion 4002 and the scan line driver circuit 4004 providedover the first substrate 4001 include a plurality of transistors. FIG.6A illustrates the transistor 4040 included in the pixel portion 4002and the transistor 4011 included in the scan line driver circuit 4004,as an example. FIG. 6B illustrates the transistor 4010 included in thepixel portion 4002 and the transistor 4011 included in the scan linedriver circuit 4004. In FIG. 6A, an insulating film 4020 is providedover the transistors 4040 and 4011, and in FIG. 6B, insulating films4020 and 4021 are provided over the transistors 4010 and 4011. Note thatan insulating film 4023 is an insulating film functioning as a basefilm.

As the transistor 4011 included in the scan line driver circuit 4004,any of the transistors having a buried channel described in Embodiment 1or Embodiment 2 can be used. The transistor having a buried channel hasexcellent on characteristics (e.g., on-state current and filed-effectmobility), which enables high-speed operation and high-speed response ofthe scan line driver circuit 4004. Described in this embodiment is anexample in which a transistor having a structure similar to that of thetransistor described in Embodiment 1 is used.

Each of the transistors 4010 and 4040 included in the pixel portion 4002is electrically connected to a display element to form a display panel.There is no particular limitation on the kind of the display element aslong as display can be performed, and various kinds of display elementscan be employed.

A buried channel is not necessarily provided for the transistor 4040included in the pixel portion 4002; thus, the transistor 4040 in which asingle layer of an oxide semiconductor layer is used for a channelformation region is provided. The transistor 4040 can be manufacturedthrough the same steps as the transistor 4011 without an increase in thenumber of manufacturing steps. An oxide semiconductor layer of thetransistor 4040 can be formed through the same steps as the third oxidesemiconductor layer of the transistor 4011. In the case where thedisplay device does not have a large size, the transistor 4040 does notparticularly need to have improved on characteristics. With a singleoxide semiconductor layer, the off-state current of the transistor 4040can be smaller than that of the transistor 4011, whereby a displaydevice with low power consumption can be obtained.

An example of a liquid crystal display device using a liquid crystalelement as a display element is illustrated in FIG. 6A. In FIG. 6A, aliquid crystal element 4013 which is a display element includes thefirst electrode layer 4030, the second electrode layer 4031, and aliquid crystal layer 4008. Insulating films 4032 and 4033 serving asalignment films are provided so that the liquid crystal layer 4008 isinterposed therebetween. The second electrode layer 4031 is provided onthe second substrate 4006 side, and the first electrode layer 4030 andthe second electrode layer 4031 are stacked, with the liquid crystallayer 4008 interposed therebetween.

A columnar spacer denoted by reference numeral 4035 is obtained byselective etching of an insulating film and is provided in order tocontrol the thickness (a cell gap) of the liquid crystal layer 4008.Alternatively, a spherical spacer may be used.

In the case where a liquid crystal element is used as the displayelement, a thermotropic liquid crystal, a low-molecular liquid crystal,a high-molecular liquid crystal, a polymer dispersed liquid crystal, aferroelectric liquid crystal, an anti-ferroelectric liquid crystal, orthe like can be used. Such a liquid crystal material (liquid crystalcomposition) exhibits a cholesteric phase, a smectic phase, a cubicphase, a chiral nematic phase, an isotropic phase, or the like dependingon conditions.

Alternatively, a liquid crystal composition exhibiting a blue phase forwhich an alignment film is unnecessary may be used for the liquidcrystal layer 4008. A blue phase is one of liquid crystal phases, whichis generated just before a cholesteric phase changes into an isotropicphase while temperature of cholesteric liquid crystal is increased. Theblue phase can be exhibited using a liquid crystal composition which isa mixture of a liquid crystal and a chiral agent. In order to increasethe temperature range where the blue phase is exhibited, a liquidcrystal layer may be formed by adding a polymerizable monomer, apolymerization initiator, and the like to a liquid crystal compositioncapable of exhibiting a blue phase and by performing polymerstabilization treatment. The liquid crystal composition exhibiting ablue phase has a short response time, and has optical isotropy, whichcontributes to the exclusion of the alignment process and reduction ofviewing angle dependence. In addition, since an alignment film does notneed to be provided and rubbing treatment is unnecessary, electrostaticdischarge damage caused by the rubbing treatment can be prevented anddefects and damage of the liquid crystal display device can be reducedin the manufacturing process. Thus, productivity of the liquid crystaldisplay device can be increased. A transistor including an oxidesemiconductor film has a possibility that the electric characteristicsof the transistor may vary significantly by the influence of staticelectricity and deviate from the designed range. Therefore, it is moreeffective to use a liquid crystal composition exhibiting a blue phasefor the liquid crystal display device including the transistor thatincludes the oxide semiconductor film.

The specific resistivity of the liquid crystal material is higher thanor equal to 1×10⁹ Ω·cm, preferably higher than or equal to 1×10¹¹ Ω·cm,further preferably higher than or equal to 1×10¹² Ω·cm. Note that thespecific resistivity in this specification is measured at 20° C.

The size of storage capacitor formed in the liquid crystal displaydevice is set considering the leakage current of the transistor providedin the pixel portion or the like so that charge can be held for apredetermined period. The size of the storage capacitor may be setconsidering the off-state current of a transistor or the like. By usingthe transistor including the oxide semiconductor film disclosed in thisspecification, it is enough to provide a storage capacitor havingcapacitance that is ⅓ or less, preferably ⅕ or less of liquid crystalcapacitance of each pixel.

In the transistor 4040 including the oxide semiconductor film disclosedin this specification, the current in an off state (off-state current)can be controlled to be small. Accordingly, an electrical signal such asan image signal can be held for a longer period in the pixel, and awriting interval can be set longer in an on state. Accordingly,frequency of refresh operation can be reduced, which leads to an effectof suppressing power consumption.

Further, in the transistor 4011 including the oxide semiconductor filmdisclosed in this specification, the field-effect mobility can becontrolled to be high; thus, the scan line driver circuit 4004 can driveat high speed. According to this embodiment, a switching transistor in apixel portion and a driver transistor in a driver circuit portion can beformed over one substrate. That is, since a semiconductor device formedusing a silicon wafer or the like is not additionally needed for adriver circuit, the number of components of the semiconductor device canbe reduced.

In addition, a transistor including the same stack of oxidesemiconductor layers as the transistor 4011 may be used for a pixelportion. By using a transistor which can operate at high speed in thepixel portion, a high-quality image or a large-sized display can also beprovided.

For the liquid crystal display device, a twisted nematic (TN) mode, anin-plane-switching (IPS) mode, a fringe field switching (FFS) mode, anaxially symmetric aligned micro-cell (ASM) mode, an optical compensatedbirefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, anantiferroelectric liquid crystal (AFLC) mode, or the like can be used.

A normally black liquid crystal display device such as a transmissiveliquid crystal display device utilizing a vertical alignment (VA) modeis preferable. Some examples are given as the vertical alignment mode.For example, a multi-domain vertical alignment (MVA) mode, a patternedvertical alignment (PVA) mode, an Advanced Super View (ASV) mode, andthe like can be used. Furthermore, this embodiment can be applied to aVA liquid crystal display device. The VA liquid crystal display devicehas a kind of form in which alignment of liquid crystal molecules of aliquid crystal display panel is controlled. In the VA liquid crystaldisplay device, liquid crystal molecules are aligned in a verticaldirection with respect to a panel surface when no voltage is applied.Moreover, it is possible to use a method called domain multiplication ormulti-domain design, in which a pixel is divided into some regions(subpixels) and molecules are aligned in different directions in theirrespective regions.

In the display device, a black matrix (a light-blocking layer), anoptical member (an optical substrate) such as a polarizing member, aretardation member, or an anti-reflection member, and the like areprovided as appropriate. For example, circular polarization may beobtained by using a polarizing substrate and a retardation substrate. Inaddition, a backlight, a side light, or the like may be used as a lightsource.

As a display method in the pixel portion, a progressive method, aninterlace method or the like can be employed. Further, color elementscontrolled in a pixel at the time of color display are not limited tothree colors: R, G, and B (R, G, and B correspond to red, green, andblue, respectively). For example, R, G, B, and W (W corresponds towhite); R, G, B, and one or more of yellow, cyan, magenta, and the like;or the like can be used. Further, the sizes of display regions may bedifferent between respective dots of color elements. Note that thedisclosed invention is not limited to the application to a displaydevice for color display; the disclosed invention can also be applied toa display device for monochrome display.

Alternatively, as the display element included in the display device, alight-emitting element utilizing electroluminescence can be used.Light-emitting elements utilizing electroluminescence are classifiedaccording to whether a light-emitting material is an organic compound oran inorganic compound. In general, the former is referred to as anorganic EL element, and the latter is referred to as an inorganic ELelement.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a layer containing a light-emitting organic compound,and current flows. The carriers (electrons and holes) are recombined,and thus, the light-emitting organic compound is excited. Thelight-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. Owing to such a mechanism, thislight-emitting element is referred to as a current-excitationlight-emitting element. Note that an example of an organic EL element asa light-emitting element is described here.

In order to extract light emitted from the light-emitting element, it isacceptable as long as at least one of a pair of electrodes has alight-transmitting property. A transistor and a light-emitting elementare formed over a substrate. The light-emitting element can have a topemission structure in which light emission is extracted through thesurface opposite to the substrate; a bottom emission structure in whichlight emission is extracted through the surface on the substrate side;or a dual emission structure in which light emission is extractedthrough the surface opposite to the substrate and the surface on thesubstrate side, and a light-emitting element having any of theseemission structures can be used.

An example of a light-emitting device in which a light-emitting elementis used as a display element is illustrated in FIG. 6B. A light-emittingelement 4513 which is a display element is electrically connected to thetransistor 4010 provided in the pixel portion 4002. A structure of thelight-emitting element 4513 is not limited to the stacked-layerstructure including the first electrode layer 4030, anelectroluminescent layer 4511, and the second electrode layer 4031. Thestructure of the light-emitting element 4513 can be changed asappropriate depending on a direction in which light is extracted fromthe light-emitting element 4513, or the like.

A partition wall 4510 can be formed using an organic insulating materialor an inorganic insulating material. It is particularly preferable thatthe partition wall 4510 be formed using a photosensitive resin materialto have an opening portion over the first electrode layer 4030 so that asidewall of the opening portion is formed as a tilted surface withcontinuous curvature.

The electroluminescent layer 4511 may be formed using either a singlelayer or a plurality of layers stacked.

A protective film may be formed over the second electrode layer 4031 andthe partition wall 4510 in order to prevent entry of oxygen, hydrogen,moisture, carbon dioxide, or the like into the light-emitting element4513. As the protective film, a silicon nitride film, a silicon nitrideoxide film, a DLC film, or the like can be formed.

Further, a layer containing an organic compound may be deposited by adeposition method to cover the light-emitting element 4513 so thatoxygen, hydrogen, moisture, carbon dioxide, and the like do not enterthe light-emitting element 4513.

In addition, in a space which is formed with the first substrate 4001,the second substrate 4006, and the sealant 4005, a filler 4514 isprovided for sealing. It is preferable that a panel be packaged (sealed)with a protective film (such as a laminate film or an ultravioletcurable resin film) or a cover material with high air-tightness andlittle degasification so that the panel is not exposed to the outsideair, in this manner.

As the filler 4514, an ultraviolet curable resin or a thermosettingresin can be used as well as an inert gas such as nitrogen or argon. Forexample, polyvinyl chloride (PVC), an acrylic resin, a polyimide resin,an epoxy resin, a silicone resin, polyvinyl butyral (PVB), or ethylenevinyl acetate (EVA) can be used. For example, nitrogen is used for thefiller.

In addition, if needed, an optical film, such as a polarizing plate, acircularly polarizing plate (including an elliptically polarizingplate), a retardation plate (a quarter-wave plate or a half-wave plate),or a color filter, may be provided as appropriate on a light-emittingsurface of the light-emitting element. Further, the polarizing plate orthe circularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by projections and depressions on the surface so as to reducethe glare can be performed.

Further, an electronic paper in which electronic ink is driven can beprovided as the display device. The electronic paper is also referred toas an electrophoretic display device (an electrophoretic display) and isadvantageous in that it has the same level of readability as plainpaper, it has lower power consumption than other display devices, and itcan be made thin and lightweight.

An electrophoretic display device can have various modes. Anelectrophoretic display device contains a plurality of microcapsulesdispersed in a solvent or a solute, each microcapsule containing firstparticles which are positively charged and second particles which arenegatively charged. By applying an electric field to the microcapsules,the particles in the microcapsules move in opposite directions to eachother and only the color of the particles gathering on one side isdisplayed. Note that the first particles and the second particles eachcontain pigment and do not move without an electric field. Moreover, thefirst particles and the second particles have different colors (whichmay be colorless).

Thus, an electrophoretic display device is a display device thatutilizes a so-called dielectrophoretic effect by which a substancehaving a high dielectric constant moves to a high-electric field region.

A solution in which the above microcapsules are dispersed in a solventis referred to as electronic ink. This electronic ink can be printed ona surface of glass, plastic, cloth, paper, or the like. Furthermore, byusing a color filter or particles that have a pigment, color display canalso be achieved.

Note that the first particles and the second particles in themicrocapsules may each be formed of a single material selected from aconductive material, an insulating material, a semiconductor material, amagnetic material, a liquid crystal material, a ferroelectric material,an electroluminescent material, an electrochromic material, and amagnetophoretic material, or formed of a composite material of any ofthese.

As the electronic paper, a display device using a twisting ball displaysystem can be used. The twisting ball display system refers to a methodin which spherical particles each colored in black and white arearranged between a first electrode layer and a second electrode layerwhich are electrode layers used for a display element, and a potentialdifference is generated between the first electrode layer and the secondelectrode layer to control orientation of the spherical particles, sothat display is performed.

In FIGS. 5A to 5C and FIGS. 6A and 6B, as the first substrate 4001 andthe second substrate 4006, flexible substrates, for example, plasticsubstrates having a light-transmitting property or the like can be used,in addition to glass substrates. As plastic, a fiberglass-reinforcedplastics (FRP) plate, a polyvinyl fluoride (PVF) film, a polyester film,or an acrylic resin film can be used. In the case where alight-transmitting property is not needed, a metal substrate (metalfilm) of aluminum, stainless steel, or the like may be used. Forexample, a sheet with a structure in which an aluminum foil issandwiched between PVF films or polyester films may be used.

In this embodiment, an aluminum oxide film is used as the insulatingfilm 4020.

The aluminum oxide film provided as the insulating film 4020 over anoxide semiconductor film has a superior shielding effect (blockingeffect), which is not permeable to either oxygen or impurities such ashydrogen or moisture.

Therefore, during the manufacturing process and after the manufacture,the aluminum oxide film functions as a protective film for preventingentry of an impurity such as hydrogen or moisture, which can cause achange, into the oxide semiconductor film and release of oxygen, whichis a main component material of the oxide semiconductor, from the oxidesemiconductor film.

The insulating film 4021 serving as a planarizing insulating film can beformed using an organic material having heat resistance, such as anacrylic resin, a polyimide resin, a benzocyclobutene-based resin, apolyamide resin, or an epoxy resin. Other than such organic materials,it is also possible to use a low-dielectric constant material (a low-kmaterial), a siloxane-based resin, PSG (phosphosilicate glass), BPSG(borophosphosilicate glass), or the like. The insulating film may beformed by stacking a plurality of insulating films formed of thesematerials.

The first electrode layer and the second electrode layer (each of whichmay be called a pixel electrode layer, a common electrode layer, acounter electrode layer, or the like) for applying voltage to thedisplay element may have light-transmitting properties orlight-reflecting properties, which depends on the direction in whichlight is extracted, the position where the electrode layer is provided,the pattern structure of the electrode layer, and the like.

The first electrode layer 4030 and the second electrode layer 4031 canbe formed using a light-transmitting conductive material such as indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide, indium zinc oxide, indiumtin oxide to which silicon oxide is added, or graphene.

The first electrode layer 4030 and the second electrode layer 4031 eachcan be formed using one kind or plural kinds selected from metal such astungsten (W), molybdenum (Mo), zirconium (Zr), hafnium (Hf), vanadium(V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt (Co), nickel(Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper (Cu), orsilver (Ag); an alloy thereof; and a nitride thereof.

Since the transistor is easily broken owing to static electricity or thelike, a protective circuit for protecting the driver circuit ispreferably provided. The protection circuit is preferably formed using anonlinear element.

By using any of the transistors described in Embodiment 1 or Embodiment2 as described above, the semiconductor device can have a variety offunctions.

Embodiment 4

A semiconductor device having an image sensor function of readinginformation on an object can be formed with use of the transistordescribed in Embodiment 1 or Embodiment 2.

An example of a semiconductor device having an image sensor function isillustrated in FIG. 7A. FIG. 7A is an equivalent circuit diagram of aphoto sensor, and FIG. 7B is a cross-sectional view illustrating part ofthe photo sensor.

In a photodiode 602, one electrode is electrically connected to aphotodiode reset signal line 658, and the other electrode iselectrically connected to a gate of a transistor 640. One of a sourceand a drain of the transistor 640 is electrically connected to a photosensor reference signal line 672, and the other of the source and thedrain thereof is electrically connected to one of a source and a drainof a transistor 656. A gate of the transistor 656 is electricallyconnected to a gate signal line 659, and the other of the source and thedrain thereof is electrically connected to a photo sensor output signalline 671.

Note that in circuit diagrams in this specification, a transistorincluding an oxide semiconductor film is denoted by a symbol “OS” sothat it can be identified as a transistor including an oxidesemiconductor film. As each of the transistor 640 and the transistor 656in FIG. 7A, any of the transistors described in Embodiment 1 orEmbodiment 2 can be employed, which is a transistor including an oxidesemiconductor stack. Described in this embodiment is an example in whicha transistor having a structure similar to that of the transistordescribed in Embodiment 1 is used.

FIG. 7B is a cross-sectional view of the photodiode 602 and thetransistor 640 in the photo sensor. The photodiode 602 functioning as asensor and the transistor 640 are provided over a substrate 601 (a TFTsubstrate) having an insulating surface. A substrate 613 is providedover the photodiode 602 and the transistor 640 with an adhesive layer608 interposed therebetween.

An insulating film 631, an insulating film 632, an interlayer insulatingfilm 633, and an interlayer insulating film 634 are provided over thetransistor 640. The photodiode 602 is provided over the interlayerinsulating film 633. In the photodiode 602, a semiconductor film 606 a,a semiconductor film 606 b, and a third semiconductor film 606 c aresequentially stacked from the interlayer insulating film 633 side,between the electrode layer 641 formed over the interlayer insulatingfilm 633 and the electrode layer 642 formed over the interlayerinsulating film 634.

The electrode layer 641 is electrically connected to a conductive layer643 provided for the interlayer insulating film 634, and the electrodelayer 642 is electrically connected to a conductive layer 645 throughthe electrode layer 641. The conductive layer 645 is electricallyconnected to a gate electrode layer of the transistor 640, and thephotodiode 602 is electrically connected to the transistor 640.

Here, a pin photodiode in which a semiconductor film having p-typeconductivity as the first semiconductor film 606 a, a high-resistancesemiconductor film (i-type semiconductor film) as the secondsemiconductor film 606 b, and a semiconductor film having n-typeconductivity as the third semiconductor film 606 c are stacked isillustrated as an example.

The first semiconductor film 606 a is a p-type semiconductor film andcan be formed using an amorphous silicon film containing an impurityelement imparting p-type conductivity. The first semiconductor film 606a is formed by a plasma CVD method with use of a semiconductor sourcegas containing an impurity element belonging to Group 13 (e.g., boron(B)). As the semiconductor material gas, silane (SiH₄) may be used.Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like may beused. Further alternatively, an amorphous silicon film which does notcontain an impurity element may be formed, and then, an impurity elementmay be introduced to the amorphous silicon film with use of a diffusionmethod or an ion injecting method. Heating or the like may be conductedafter introducing the impurity element by an ion implantation method orthe like in order to diffuse the impurity element. In this case, as amethod of forming the amorphous silicon film, an LPCVD method, achemical vapor deposition method, a sputtering method, or the like maybe used. The first semiconductor film 606 a is preferably formed to havea thickness greater than or equal to 10 nm and less than or equal to 50nm.

The second semiconductor film 606 b is an i-type semiconductor film(intrinsic semiconductor film) and is formed using an amorphous siliconfilm. As for formation of the second semiconductor film 606 b, anamorphous silicon film is formed by a plasma CVD method with use of asemiconductor source gas. As the semiconductor source gas, silane (SiH₄)may be used. Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or thelike may be used. The second semiconductor film 606 b may be formed byan LPCVD method, a vapor deposition method, a sputtering method, or thelike. The second semiconductor film 606 b is preferably formed to have athickness greater than or equal to 200 nm and less than or equal to 1000nm.

The third semiconductor film 606 c is an n-type semiconductor film andis formed using an amorphous silicon film containing an impurity elementimparting n-type conductivity. The third semiconductor film 606 c isformed by a plasma CVD method with use of a semiconductor source gascontaining an impurity element belonging to Group 15 (e.g., phosphorus(P)). As the semiconductor source gas, silane (SiH₄) may be used.Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like may beused. Further alternatively, an amorphous silicon film which does notcontain an impurity element may be formed, and then, an impurity elementmay be introduced to the amorphous silicon film with use of a diffusionmethod or an ion implantation method. Heating or the like may beconducted after introducing the impurity element by an ion implantationmethod or the like in order to diffuse the impurity element. In thiscase, as a method for forming the amorphous silicon film, an LPCVDmethod, a chemical vapor deposition method, a sputtering method, or thelike may be used. The third semiconductor film 606 c is preferablyformed to have a thickness greater than or equal to 20 nm and less thanor equal to 200 nm.

The first semiconductor film 606 a, the second semiconductor film 606 b,and the third semiconductor film 606 c are not necessarily an amorphoussemiconductor, but may be a polycrystalline semiconductor or amicrocrystalline semiconductor (semi-amorphous semiconductor: SAS).

In addition, the mobility of holes generated by the photoelectric effectis lower than the mobility of electrons. Therefore, a pin photodiode hasbetter characteristics when a surface on the p-type semiconductor filmside is used as a light-receiving plane. Here, an example in which lightreceived by the photodiode 602 from a surface of the substrate 601, overwhich the pin photodiode is formed, is converted into electric signalsis described. Further, light from the semiconductor film having aconductivity type opposite to that of the semiconductor film on thelight-receiving plane is disturbance light; therefore, the electrodelayer is preferably formed using a light-blocking conductive film. Notethat the n-type semiconductor film side may alternatively be alight-receiving plane.

With use of an insulating material, the insulating film 632, theinterlayer insulating film 633, and the interlayer insulating film 634can be formed, depending on the material, use of a method or a tool(equipment) such as a sputtering method, a plasma CVD method, an SOGmethod, spin coating, dipping, spray coating, a droplet discharge method(such as an inkjet method), a printing method (such as screen printingor offset printing), a doctor knife, a roll coater, a curtain coater, ora knife coater.

In this embodiment, an aluminum oxide film is used as the insulatingfilm 631. The insulating film 631 can be formed by a sputtering methodor a plasma CVD method.

The aluminum oxide film provided as the insulating film 631 over theoxide semiconductor film has a superior shielding effect (blockingeffect), which is not permeable to either oxygen or impurities such ashydrogen or moisture.

Therefore, during the manufacturing process and after the manufacture,the aluminum oxide film functions as a protective film for preventingentry of an impurity such as hydrogen or moisture, which might causevariation in characteristics, into the oxide semiconductor film, andrelease of oxygen, which is a main component material of the oxidesemiconductor, from the oxide semiconductor film.

The insulating film 632 can be formed using an inorganic insulatingmaterial and can have a single-layer structure or a stacked-layerstructure including any of oxide insulating films such as a siliconoxide layer, a silicon oxynitride layer, an aluminum oxide layer, and analuminum oxynitride layer; and nitride insulating films such as asilicon nitride layer, a silicon nitride oxide layer, an aluminumnitride layer, and an aluminum nitride oxide layer.

For a reduction in surface roughness, an insulating film functioning asa planarization insulating film is preferably used as each of theinterlayer insulating films 633 and 634. For the interlayer insulatingfilms 633 and 634, for example, an organic insulating material havingheat resistance, such as a polyimide resin, an acrylic resin, abenzocyclobutene-based resin, a polyamide resin, or an epoxy resin, canbe used. Other than such organic insulating materials, it is possible touse a single layer or stacked layers of a low-dielectric constantmaterial (a low-k material), a siloxane-based resin, phosphosilicateglass (PSG), borophosphosilicate glass (BPSG), or the like.

With detection of light that enters the photodiode 602, data on anobject to be detected can be read. Note that a light source such as abacklight can be used at the time of reading data on an object.

As described above, with use of the oxide semiconductor stack includinga buried channel, electric characteristics of the transistor can becontrolled accurately, and desired electric characteristics can be givento the transistor. Therefore, with use of such a transistor, asemiconductor device which meets various purposes such as highperformance, high reliability, and low power consumption can beprovided.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

Embodiment 5

A semiconductor device disclosed in this specification can be applied toa variety of electronic devices (including game machines). Examples ofelectronic devices to which the present invention can be applied includea television set (also referred to as a television or a televisionreceiver), a monitor of a computer, cameras such as a digital camera anda digital video camera, a digital photo frame, a mobile phone, aportable game machine, a portable information terminal, an audioreproducing device, a game machine (e.g., a pachinko machine or a slotmachine), a housing of a game machine, and the like. Specific examplesof such electronic devices are illustrated in FIGS. 8A to 8D.

FIG. 8A illustrates a table 9000 having a display portion. In the table9000, a display portion 9003 is incorporated into a housing 9001. Asemiconductor device manufactured according to one embodiment of thepresent invention can be used for the display portion 9003, and an imagecan be displayed on the display portion 9003. Note that the housing 9001is supported by four leg portions 9002. Further, a power cord 9005 forsupplying power is provided for the housing 9001.

The display portion 9003 has a touch-input function. When a user touchesdisplayed buttons 9004 which are displayed on the display portion 9003of the table 9000 with his/her fingers or the like, the user can carryout operation of the screen and input of information. Further, when thetable may be made to communicate with home appliances or control thehome appliances, the display portion 9003 may function as a controldevice which controls the home appliances by operation on the screen.For example, with use of the semiconductor device having an image sensordescribed in Embodiment 3, the display portion 9003 can function as atouch panel.

Further, the screen of the display portion 9003 can be placedperpendicular to a floor with a hinge provided for the housing 9001;thus, the table 9000 can also be used as a television set. A televisionset with a large screen takes up too much space that is available in asmall room. However, with a table having a display portion therein, itis possible to make the use of the space in the room.

FIG. 8B illustrates a television set 9100. In the television set 9100, adisplay portion 9103 is incorporated in a housing 9101. A semiconductordevice manufactured using one embodiment of the present invention can beused in the display portion 9103, so that an image can be displayed onthe display portion 9103. Note that the housing 9101 is supported by astand 9105 here.

The television set 9100 can be operated with an operation switch of thehousing 9101 or a separate remote controller 9110. Channels and volumecan be controlled with an operation key 9109 of the remote controller9110 so that an image displayed on the display portion 9103 can becontrolled. Furthermore, the remote controller 9110 may be provided witha display portion 9107 for displaying data output from the remotecontroller 9110.

The television set 9100 illustrated in FIG. 8B is provided with areceiver, a modem, and the like. With the receiver, the television set9100 can receive a general television broadcast. Further, when thetelevision set 9100 is connected to a communication network with orwithout wires connection via the modem, one-way (from a transmitter to areceiver) or two-way (between a transmitter and a receiver or betweenreceivers) data communication can be performed.

When the semiconductor device including a buried channel described inthe above embodiment is used for the display portion 9103 of thetelevision set, the television set can have higher display quality thana conventional one.

FIG. 8C illustrates a computer which includes a main body 9201, ahousing 9202, a display portion 9203, a keyboard 9204, an externalconnection port 9205, a pointing device 9206, and the like. The computerincludes a semiconductor device manufactured according to one embodimentof the present invention for the display portion 9203.

Further, when the semiconductor device described in the above embodimentis used for the display portion 9203 of the computer, the displayportion can have higher display quality than a conventional one.

FIG. 8D illustrates an example of a mobile phone. A mobile phone 9500 isprovided with a display portion 9502 incorporated in a housing 9501, anoperation button 9503, an external connection port 9504, a speaker 9505,a microphone 9506, and the like. Note that the mobile phone 9500 ismanufactured using a semiconductor device manufactured using oneembodiment of the present invention for the display portion 9502.

Users can input data, make a call, or text a message by touching thedisplay portion 9502 of the mobile phone 9500 illustrated in FIG. 8Dwith their fingers or the like.

There are mainly three screen modes for the display portion 9502. Thefirst mode is a display mode mainly for displaying images. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is the one in which two modes of the display mode and the inputmode are combined.

For example, in the case of making a call or text messaging, a textinput mode mainly for inputting text is selected for the display portion9502 so that characters displayed on a screen can be input. In thiscase, it is preferable to display a keyboard or number buttons on almostthe entire screen of the display portion 9502.

By providing a detection device which includes a sensor for detectinginclination, such as a gyroscope or an acceleration sensor, inside themobile phone 9500, the direction of the mobile phone 9500 (whether themobile phone 9500 is placed horizontally or vertically for a landscapemode or a portrait mode) is determined so that display on the screen ofthe display portion 9502 can be automatically switched.

In addition, the screen mode is switched by touching the display portion9502 or operating the operation button 9503 of the housing 9501.Alternatively, the screen modes can be switched depending on kinds ofimages displayed in the display portion 9502. For example, when a signalof an image displayed on the display portion is a signal of moving imagedata, the screen mode is switched to the display mode. When the signalis a signal of text data, the screen mode is switched to the input mode.

Moreover, in the input mode, when input by touching the display portion9502 is not performed within a specified period of time while a signaldetected by an optical sensor in the display portion 9502 is detected,the screen mode may be controlled so as to be switched from the inputmode to the display mode.

The display portion 9502 can also function as an image sensor. Forexample, an image of a palm print, a fingerprint, or the like is takenby touching the display portion 9502 with the palm or the finger,whereby personal authentication can be performed. Further, by providinga backlight or a sensing light source which emits a near-infrared lightin the display portion, an image of a finger vein, a palm vein, or thelike can be taken.

When the semiconductor device described in the above embodiments isemployed, color mixing, color shift, or the like in display does noteasily occur. Therefore, with use of the semiconductor device for thedisplay portion 9502 of the mobile phone, the mobile phone can havehigher display quality than a conventional one. In addition, the pair ofsubstrates is supported by the light-blocking spacer, so that thesemiconductor device is extremely resistant to external force such asimpact, distortion, or the like. Thus, the semiconductor device can befavorably used for the mobile phone illustrated in FIG. 8D.

The methods and structures described in this embodiment can be combinedas appropriate with any of the methods and structures described in theother embodiments.

Example 1

In this example, samples including a first oxide semiconductor layer, asecond oxide semiconductor layer with a smaller energy gap than thefirst oxide semiconductor layer, and a third oxide semiconductor layer,which are stacked in this order, was manufactured; ionization potentialof the sample were measured; and the energy band was calculated based onthe result and an energy bad diagram was obtained. In thisspecification, the value of the ionization potential corresponds to thesum of the band gap and the electron affinity, and the value of the bandgap is a value obtained by measuring a single material film with anellipsometer.

As Sample 1, a 5-nm-thick IGZO film, a 5-nm-thick In—Sn—Zn-based oxidefilm, and a 5-nm-thick IGZO film were stacked over a single crystalsilicon substrate. These films were deposited by a sputtering method ata substrate temperature of 300° C. in an oxygen atmosphere (theproportion of oxygen is 100%). An oxide target having a compositionratio, In:Ga:Zn=1:1:1 [atomic ratio] was used as the target to form theIGZO film. Further, an oxide target having a composition ratio,In:Sn:Zn=2:1:3 [atomic ratio] was used for the In—Sn—Zn-based oxidefilm.

FIG. 11A is a TEM image of a cross-section of Sample 2 obtained bystacking a 5-nm-thick first IGZO film 1001, a 5-nm-thick In—Sn—Zn-basedoxide film 1002, and a 5-nm-thick second IGZO film 1003 to be stackedover a quartz substrate 1000 under the same deposition conditions as theabove, with the use of a quartz substrate. Note that a schematic viewthereof is illustrated in FIG. 11B. In FIG. 11B, interfaces between theoxide semiconductor layers are schematically denoted by a dotted line.In some cases, the interfaces between the oxide semiconductor layers areunclear depending on the material, the deposition conditions, or heattreatment. The interfaces between the IGZO films and the In—Sn—Zn-basedoxide film can be observed in Sample 2 shown in FIG. 11A. Further, fromFIG. 11A, crystals can be observed in the second IGZO film 1003 and theIn—Sn—Zn-based oxide film 1002, and the second IGZO film 1003 and theIn—Sn—Zn-based oxide film 1002 are a c-axis aligned crystal oxidesemiconductor (CAAC-OS) film. In addition, from FIG. 11A, the first IGZOfilm 1001 has an amorphous structure. Note that in FIG. 11A, two out ofthe three layers are oxide semiconductor films having a crystallinestructure; however, the structure of the oxide semiconductor stack isnot limited thereto. Only the second IGZO film 1003 may have acrystalline structure, or all of the three layers may have an amorphousstructure.

Ionization potential was measured by ultraviolet photoelectronspectroscopy (UPS) while Sample 1 was sputtered from a surface side ofSample 1. The measurement result is shown in FIG. 9.

In FIG. 9, the horizontal axis indicates the time of sputteringconducted from the surface side of Sample 1 and the vertical axisindicates the ionization potential. Borders between the films areindicated on the assumption that the sputtering rate of the IGZO film isequal to that of the In—Sn—Zn-based oxide film. From FIG. 9, theionization potential of the In—Sn—Zn-based oxide film which wassandwiched between the IGZO films is lowered. Note that ionizationpotential corresponds to energy difference between a vacuum level andvalence band.

The energy of the conduction band was obtained by subtracting the bandgap measured by an ellipsometer from the value of the ionizationpotential, and the band structure of the stacked films was formed. Notethat the band gap of the IGZO film was 3.2 eV, and the band gap of theIn—Sn—Zn-based oxide film was 2.8 eV. FIG. 10 shows the result. FIG. 10indicates that a buried channel is formed as in the case of the energyband diagram of FIG. 1D.

According to this example, as an energy band diagram, FIG. 10 or FIG. 1Dcan be applied to the stacked layer in which IGZO films are used for thefirst oxide semiconductor layer and the third oxide semiconductor layer,and an In—Sn—Zn-based oxide film is used for the second oxidesemiconductor layer having higher ionization potential and smallerenergy gap than the first oxide semiconductor layer and the third oxidesemiconductor layer. The combination of materials of the first oxidesemiconductor layer, the second oxide semiconductor layer, and the thirdoxide semiconductor layer are not particularly limited, and inconsideration of energy gap of materials to be used, the materials maybe appropriately selected and combined by practitioners in order tosatisfy the energy band diagram shown in FIG. 10 or FIG. 1D. Forexample, a stacked structure in which IGZO films are used for the firstoxide semiconductor layer and the third oxide semiconductor layer and anIZO film is used for the second oxide semiconductor layer may be used.

EXPLANATION OF REFERENCE

101: oxide semiconductor layer, 102: oxide semiconductor layer, 103:oxide semiconductor layer, 111: oxygen-excess region, 112: oxygen-excessregion, 113: oxygen-excess region, 121 a: first low-resistance region,121 b: first low-resistance region, 121 c: channel formation region, 121d: first region, 121 e: first region, 122 a: second low-resistanceregion, 122 b: second low-resistance region, 122 c: channel formationregion, 122 d: second region, 122 e: second region, 123 a: thirdlow-resistance region, 123 b: third low-resistance region, 123 c:channel formation region, 123 d: third region, 123 e: third region, 131d: first region, 131 e: first region, 132 d: second region, 132 e:second region, 133 d: third region, 400: substrate, 401: gate electrodelayer, 402: gate insulating film, 403: oxide semiconductor stack, 405 a:source electrode layer, 405 b: drain electrode layer, 405 c: sourceelectrode layer, 405 d: drain electrode layer, 407: insulating film,421: dopant, 431: oxygen, 436: oxide insulating film, 442: gateinsulating film, 465 a: wiring layer, 465 b: wiring layer, 510:transistor, 520: transistor, 530: transistor, 540: transistor, 550:transistor, 560: transistor, 570: transistor, 601: substrate, 602:photodiode, 606 a: semiconductor film, 606 b: semiconductor film, 606 c:semiconductor film, 608: adhesive layer, 613: substrate, 631: insulatingfilm, 632: insulating film, 633: interlayer insulating film, 634:interlayer insulating film, 640: transistor, 641: electrode layer, 642:electrode layer, 643: conductive layer, 645: conductive layer, 656:transistor, 658: photodiode reset signal line, 659: gate signal line,671: photo sensor output signal line, 672: photo sensor reference signalline, 1000: quartz substrate, 1001: first IGZO film, 1002:In—Sn—Zn-based oxide film, 1003: second IGZO film, 4001: substrate,4002: pixel portion, 4003: signal line driver circuit, 4004: scan linedriver circuit, 4005: sealant, 4006: substrate, 4008: liquid crystallayer, 4010: transistor, 4011: transistor, 4013: liquid crystal element,4015: connection terminal electrode, 4016: terminal electrode, 4018:FPC, 4019: anisotropic conductive film, 4020: insulating film, 4021:insulating film, 4023: insulating film, 4030: electrode layer, 4031:electrode layer, 4032: insulating film, 4040: transistor, 4510:partition wall, 4511: electroluminescent layer, 4513: light-emittingelement, 4514: filler, 9000: table, 9001: housing, 9002: leg portion,9003: display portion, 9004: displayed button, 9005: power cord, 9100:television set, 9101: housing, 9103: display portion, 9105: stand, 9107:display portion, 9109: operation key, 9110: separate remote controller,9201: main body, 9202: housing, 9203: display portion, 9204: keyboard,9205: external connection port, 9206: pointing device, 9500: mobilephone, 9501: housing, 9502: display portion, 9503: operation button,9504: external connection port, 9505: speaker, 9506: microphone

This application is based on Japanese Patent Application Serial No.2011-135365 filed with Japan Patent Office on Jun. 17, 2011, the entirecontents of which are hereby incorporated by reference.

The invention claimed is:
 1. A semiconductor device comprising: an oxidesemiconductor stack including a first oxide semiconductor layer, asecond oxide semiconductor layer in contact with a top surface of thefirst oxide semiconductor layer, and a third oxide semiconductor layerin contact with a top surface and a side surface of the second oxidesemiconductor layer and in contact with a side surface of the firstoxide semiconductor layer, wherein the second oxide semiconductor layerhas a smaller energy gap than the first oxide semiconductor layer, andthe third oxide semiconductor layer has a larger energy gap than thesecond oxide semiconductor layer; a source electrode layer and a drainelectrode layer adjacent to the oxide semiconductor stack; a gateinsulating film adjacent to the oxide semiconductor stack; and a gateelectrode layer adjacent to the oxide semiconductor stack with the gateinsulating film interposed therebetween, wherein, in the oxidesemiconductor stack, a region which does not overlap the sourceelectrode layer or the drain electrode layer has a higher oxygenconcentration than a region which overlaps the source electrode layer orthe drain electrode layer.
 2. The semiconductor device according toclaim 1, wherein an electron affinity of the second oxide semiconductorlayer is higher than electron affinities of the first oxidesemiconductor layer and the third oxide semiconductor layer.
 3. Thesemiconductor device according to claim 1, wherein a region which is inthe oxide semiconductor stack and does not overlap with the gateelectrode layer contains dopant.
 4. The semiconductor device accordingto claim 1, further comprising: an interlayer insulating film over thegate electrode layer, wherein the interlayer insulating film has acontact hole reaching at least one of the source electrode layer and thedrain electrode layer; and a wiring layer over the interlayer insulatingfilm, wherein the wiring layer is connected to the at least one of thesource electrode layer and the drain electrode layer through the contacthole.
 5. The semiconductor device according to claim 1, wherein thesemiconductor device is one selected from the group consisting of atable having a display portion, a television set, a computer, and amobile phone.
 6. A semiconductor device comprising: an oxidesemiconductor stack including a first oxide semiconductor layer, asecond oxide semiconductor layer in contact with a top surface of thefirst oxide semiconductor layer, and a third oxide semiconductor layerin contact with a top surface and a side surface of the second oxidesemiconductor layer and in contact with a side surface of the firstoxide semiconductor layer, wherein the second oxide semiconductor layerhas a smaller energy gap than the first oxide semiconductor layer, andthe third oxide semiconductor layer has a larger energy gap than thesecond oxide semiconductor layer; a source electrode layer and a drainelectrode layer adjacent to the oxide semiconductor stack; a gateinsulating film over the oxide semiconductor stack; and a gate electrodelayer over the oxide semiconductor stack with the gate insulating filminterposed therebetween, wherein, in the oxide semiconductor stack, aregion which does not overlap the source electrode layer or the drainelectrode layer has a higher oxygen concentration than a region whichoverlaps the source electrode layer or the drain electrode layer.
 7. Thesemiconductor device according to claim 6, wherein an electron affinityof the second oxide semiconductor layer is higher than electronaffinities of the first oxide semiconductor layer and the third oxidesemiconductor layer.
 8. The semiconductor device according to claim 6,wherein a region which is in the oxide semiconductor stack and does notoverlap with the gate electrode layer contains dopant.
 9. Thesemiconductor device according to claim 6, further comprising: aninterlayer insulating film over the gate electrode layer, wherein theinterlayer insulating film has a contact hole reaching at least one ofthe source electrode layer and the drain electrode layer; and a wiringlayer over the interlayer insulating film, wherein the wiring layer isconnected to the at least one of the source electrode layer and thedrain electrode layer through the contact hole.
 10. The semiconductordevice according to claim 6, wherein the semiconductor device is oneselected from the group consisting of a table having a display portion,a television set, a computer, and a mobile phone.